Cingular Wireless has multiple data plans options under different names and different prices.  What's the difference between them is a question often asked because the Cingular web site makes this very confusing.  So I'll do my best to summarize it here.

Data Plan Basics:
Your rate plan with the carrier is based on your (voice) phone call "air time" and how it is billed.  This rate plan does not cover anything else such as text messages, picture/multimedia messages, or internet data.  Those extras are considered add on 'features' that you can either pay per use, or you can get a feature plan that will include a bucket of those extras.  These features generally can be added or removed from the plan at will without affecting your rate plan contract.  But you should verify this when signing up.
These features should be thought of in much the same way you can add caller ID, call waiting, etc. to your home phone line.
Also note that on family talk plans, most features (such as data and messaging) only cover that 1 line, not all lines on the family talk plan unless specifically indicated.  Therefore each line must have it's own data and messaging feature add-ons.

The feature called "data" (internet data) is what I'll cover here.
Internet data on Cingular's network is transmitted completely independently and different from the way phone calls are handled. Data is billed per kilobyte (KB) used, not by amount of time.  So if you download a 40KB web page, wait 2 min, then download another 40KB web page, you're charged for 80KB of data, not 2-3 minutes of time.

Just as DSL on your home phone line handles separate from the phone, data on Cingular's network should be thought of the same way where data is independent of your voice calls.  However on Cingular's network, you can't do both data and voice at the same time with GPRS/EDGE, and can with UMTS/HSDPA.  More on these later.

Why at data plan?
You may use data on Cingular's network on a "pay per use" basis.  But doing so is extremely expensive.  1MB of data transferred cost $10 at PPU (pay per use) rates.  10MB will cost you $100.
By way of comparison, the cheapest unlimited internet data plan is $20.  That same 10MB of data will still only cost $20.
It is very easy to use more data than you intended or thought possible.  On a PDA the minimum typical use for email and occasion web browsing is 40MB to 50MB, and you can easily use 300MB or more of data when using streaming video or audio services.
50MB of data would cost $500 at pay per use rates, and 300MB would cost $3000.  By way of comparison the unlimited PDA data plan is $40 for 50MB, 300MB, or 3000MB.
It is for this reason I recommend everyone have an unlimited data plan until they have used it for a few months and determined that they are using less than 2MB per month.  Note that there are 1MB, 5MB, 10MB, 20MB, and other bucket data plans that you may purchase instead of an unlimited plan if this fits your usage more appropriately.

Cingular has basically 2 data plan categories:

1. Data Connect - Intended for full internet access with a laptop, modem, or PDA
2. MEdia Net - Intended for use with a normal phone or smart phone that is not a PDA.

Of those 2 data plan categories, there are several data plans:

• Data Connect Plans:
  1. Laptop Connect - Sold for use with connecting a laptop or PC to the internet.  This applies to laptop modem cards as well as using a phone as a modem to connect a computer to the internet (through Bluetooth or via a USB cable,) a practice known as 'tethering.'
  2. Data Connect (aka PDA connect) - Sold for use with a PDA device.
     
     Laptop Connect, Data Connect, and PDA connect are all the identical data plan with different names and prices relevant to the usage.  Other than the name and price, they all have identical functionality and are interchangeable.
      
• MEdia Net Plans:
  1. MEdia Max - These are feature bundles that include the MEdia Net data plan in addition to a messaging plan.
  2. Smartphone Connect - Sold for use with a "smart phone" that is not classified as a PDA.  An example is the Cingular 2125, and Cingular 3125.  Cingular classifies a smart phone as one that runs Windows Mobile Smartphone and has a numerical keypad, not a QWERTY keyboard (such as a Blackjack.)  This is a totally asinine method for classification but that's the way it is.
     
     MEdia Net, Smartphone Connect, MEdia Works, and MEdia Max are all the identical data plan with different names and prices relevant to the usage.  Other than the name and price, they all have identical functionality and are interchangeable.
     NOTE: Smartphone Connect is listed on the "Data Connect" page because of the way it's sold.  However it is technically identical to the MEdia Net plans, not the data connect plan.

Pricing:

This is how the unlimited versions of each data plan is priced.  For smaller data plans including only so many MB, see Cingular's web site.

Unlimited Data Plans

Plan Name	Intended Use	Price
Laptop Connect	For use with laptop modem cards OR for using a phone as modem (tethering)	$60/ month*
Data Connect (PDA Connect)	For use with PDAs	$40 /month**
MEdia Max	For use with standard phones with micro browsers	$20 /month
Smartphone Connect	For use with (non PDA) smart phones.  This plan is just another name for MEdia Max plan without included messaging.	$20 /month***

*$60/month is based on getting this plan in addition to having a voice (minutes) plan for phone service either on the same or different 'line' (or SIM."  If you are not a voice/phone customer, the rate is $80
**Data Connect is listed as $45/month for unlimited usage.  This price is based on a data only plan for a PDA with no voice (minutes) plan.  If PDA/Data connect is added as a feature to any voice plan, there is a $5 discount making it $40/month.
***Smartphone Connects is sold as a 'data connect' type plan, however it is technically more like the media net plan.

These represent the least expensive unlimited data plans of each type.  There are more expensive PDA Connect and Media Max plans that simply include more messaging.

What is the difference between the two categories of plans (Data Connect plans and Media Net plans)?

There is one and only one actual difference between the data connect plans (for laptops, PDAs, and tethering) and Media Net (for using on a phone.)  That difference is 'access points.'  The data connect plan category provisions your account for unlimited access to both access points.  The MEdia Net category of plans provisions you to only one of the two access points.

Data Plans	Access Points Provisioned
Data Connect (family of plans) Laptop Connect Data Connect PDA Connect	isp.cingular wap.cingular
MEdia Net (family of plans) Smartphone Connect MEdia Max	wap.cingular (only)

wap.cingular is the standard access point all phones and PDAs (except Blackberrys) are pre-configured to use
isp.cingular is an alternate connection to support VPN use.

There are two Cingular access points:

APN (Access Point Name)	User Name	Password	IP Address
isp.cingular	isp@cingulargprs.com -or- ispda@cingulargprs.com	CINGULAR1 CINGULAR1	Real (Public Routable) IP Address
wap.cingular	wap@cingulargprs.com	CINGULAR1	Private (NAPT) IP Address

isp.cingular
Full internet access.  Basically this is a true to life ISP internet access setting just like your cable or DSL modem.  You are given a dynamically assigned 'real world' publicly addressable IP address without the restriction of a proxy or router.  The initial purpose of this type of connection was to support all types of VPN connections.  There is a firewall on this connection that will restrict all incoming unsolicited data traffic.  Therefore you can not run a server or peer-to-peer type application on this.
Additionally, Cingular optionally supports image compression when using the (ispda) login.  This connection will reduce image quality in order to reduce data usage.  This is helpful on slower connections.

wap.cingular
Internet access through a NAPT (Network Address and Port Translation) router.  You are given a NAT (private) IP address.  NAT is a method for many devices to all share one real IP address.  This is basically like being behind your home gateway/router or a corporate firewall/router.  There are no blocked ports or IP addresses on this connection (not intentionally) and you still get full internet access.  However the router has some limitations.  One is that it will not forward "GRE pakets" necessary to support PPTP type VPN (the type of VPN built in to WindowsXP.)  However, it does support IPSec type VPN (like Cisco VPN and other popular corporate VPN clients.)

Additional Information on Settings:
When using ispda@cingulargprs.com, images you download will be 'compressed' to reduce data.  It's like using a data compression proxy and may be helpful in speeding up web surfing.
isp@cingulargprs.com represents the standard connection.

Proxy:
When using the wap.cingular access point (not available on isp.cingular,) a proxy server with the address wireless.cingular.com (port 80 or 8080) is available to use.  This proxy will gain you access to the MEdia Net web site at the URL http://device.home/ and access Cingular Video at http://cingularvideo.cingular.com/ .  Without this proxy you can not access these sites.  This proxy also compresses images you download (reducing the size) which may also optimize web page downloading.

Device and Data Settings

The information above regarding the access points is originally taken from this Cingular PDF Device and Software Settings Wireless Internet, Wireless Internet Express, & Data Connect (WAP and ISP) (or download it here if that link fails.)  This document is old and refers to GPRS data.  Since then EDGE and UMTS/HSDPA have been introduced, however everything in the PDF is still currents since the settings don't actually change between these data technologies.

3G and 2.5G are interchangeable
2.5G = GPRS and EDGE
3G = UMTS and HSDPA

Data plans are not data speed dependent.  The data plan doesn't care how or what speed you connect to the Cingular network, it just cares with what access point.  You will get GPRS, EDGE, UMTS, or HSDPA with any and all data plans as long as your phone or device is capable of doing so.  Your phone or device will connect with the fastest technology that it supports and that the network supports in that area.  If you have an HSDPA phone or PDA, you will automatically get HSDPA (or UMTS) or fall back to EDGE (or GPRS) if the signal or network can support.  The unlimited data plans will not work any differently when connected to these different types of networks.

Data plans are device independent and interchangeable:

On Cingular you have a SIM card.  That SIM card is your phone 'line'.  Any phone or device you put that SIM card in will give you the same phone number and service whether it's a flip phone, PDA, smart phone, or laptop data card.  A data plan is added to each line as a feature, so that data plan will also work just the same on any device you put the SIM in to (flip phone, PDA, smart phone, laptop card. etc.)

Therefore if you sign up for laptop connect, you can put that SIM in to any PDA, phone, or laptop modem card and get your laptop connect unlimited data.  Likewise, if you sign up for MEdia Max for the line (SIM) that you use in your RAZR, you can put that SIM in to a laptop modem card, PDA, or any phone and get your MEdia Net unlimited data access.
It's that simple

If you haven't figure it out from my last statement.  All data plans work on all phones, PDAs, and devices.  They do not make any distinction.  A Media Max plan will work on any phone, PDA, or laptop card just as a laptop data plan will work on any of these.  However, Cingular's policy is that you only use the more expensive plans with lesser devices, not cheaper plans with more capable devices.

Therefore, with a data connect plan (laptop connect, data connect, PDA connect) you can use that SIM in any phone or device without making any changes to the device configuration since that plan supports both access points.

Will using the wrong data plan on the wrong device incur penalties?
I have described how any data plan can be interchanged with any device.  Naturally one might assume that because of this you are allowed to do so.  Cingular's policy is that is "OK" to use any more expensive data plan for any less expensive use, but not the other way around.  By OK I mean by the rules of the "Terms and conditions" posted on Cingular's web site.

	$60 Laptop Connect	$40 Data Connect (PDA)	$20 MEdia Max or Smartphone Connect
Laptop Modem Card	OK	NO	NO
'Tethering' with a phone or PDA as modem	OK	NO	NO
PDA	OK	OK	NO
Phone Internal browser only	OK	OK	OK

However, the terms and conditions are not actively enforced.  Therefore if you use the Media Net plan in a laptop card, for tethering, or in a PDA, there are no active measures to bill you for additional usage, cancel your plan, or penalize you in any way.  Currently the terms and conditions state that if you are caught doing so, you risk having your data plan feature canceled from your account.

Do they know if you're using your MEdia Net SIM in a laptop card or PDA?
The answer is yes, they have the capability of knowing.  Each device has an IMEI number (unique serial number) that is sent to them when the device is on.  Based on the IMEI number they can determine what type of device you're using.  However, since they openly allow you to use your SIM in any GSM phone you chose, there are currently no restrictions for doing so which currently carries over to the data plan.  So despite the ability to know, there aren't, and haven't been, any measures to restrict or enforce use of a data plan in the 'wrong' device.

This enforcement could change.  At some point Cingular could restrict your use of your data plan to only 1 registered devices.  But as of now and the near future, there appears to be no plans for such action.

If you have any questions about what data plan is acceptable under the terms in conditions for your device, call Cingular customer care and they will be happy to tell you.

One very important piece of information you should consider when using any Media Max or Smartphone Connect with a PDA, Laptop card, or for tethering.  As I described there are two different access points.
The Cingular connect software used for laptop cards as well as 'tethering' with a phone as a modem is, by default, configured to use the isp.cingular access point.  That access point is only available for laptop connect, data connect, and PDA connect plans.
All phones, smartphones, and PDAs are, by default, set up to use the wap.cingular access point.  That access point is available on all data plans.  So note that if you got the Media Max plan, you'll be able to use it with a PDA without doing any re-configuration.

Therefore when using Cingular connect software, laptop card, or any method of tethering with a Media Max or Smartphone connect software, make sure you re-configure it to use the wap.cingular access point.