AT&T (formerly Cingular) has a prepaid SIM offers are called "GoPhone". The SIM-card and the activation is for free and takes probably 5 mins. No ID required. You only need to buy some credit at the time of activation. SIM-only is not available online. You have to stop by at an AT&T store.

Currently there arefour calling plans to choose from:

1. $50 Monthly Unlimited Plan (unlimited calls, texts and unlimited data)
   
2. $25 Monthly Plan (250mins calls, unlimited text, data extra)
   
3. $2 Daily Unlimited Plan (unlimited calls and texts, charged only on days used, data extra)
   
4. 10¢/Minute Plan Pay-per use. No daily, weekly or monthly charge (10¢/min calls, 20-25¢ text, data extra)

If you don't want to make a lot of calls or texts (<20min/day) within the US just take Option 4 and get data option:

Data options: For plan 2,3 and 4 standard data rate is 1¢ / 5kB = 2$ / MB but you can immidiately add Mobile web browsing options. Currently there are the following options:

• 10MB = $10.00 / month
• 100MB = $15.00 / month
• 500MB = $25.00 / month

The standard APN is wap.cingular and there are no extra settings needed anymore (user/pw etc...)
http://www.wireless.att.com/cell-phone-service/go-phones/index.jsp


Probably you can activate the options multiple times a month. Tested and works with unlocked iPhone in December '09 with the old 100MB/$19.99 package (but you need to Change the APN - see below).
For international calls there is also an "International Long Distance Package" tht includes 250mins to landlines in 50 countries (4 cent/min).

DataConnect Pass
In addition there are the following options (data only, no voice service alongside):

• DataConnect Day Pass (100 MB) - $15
• DataConnect Week Pass (300 MB) - $30
• DataConnect Month Pass (1 GB) - $50

For details see: www.att.com/buyasession
As of January 2011, the SIM cards for the DataConnect passes are also free. The shop just sells you the SIM card and you have to go online to the "buyasession" link and create an account. The pass is then paid with a credit card. During registration the IMEI of a device has to be entered. However, this is not checked when later using the SIM, i.e. it can be used in more than one device.
For the data connect offers the APN is isp.cingular

Misc information:

• The SIM card is only valid if not used for 3 months (previously 6 months).

Older information not related to the DataConnect option:

• AT&T stopped the unlimited plan in Nov 08.

• The options are activated by calling 611 (a voice recognition system). Select "Feature Packages" and then "MediaNet".
• Also with 611 you can check you account balance and data-option (also amount of recouring data) - select "check my feature packages"
• With USSD *777# (and Dial-Button) you can check your account balance

• According to this link, the APN to use for the standard offer is wap.cingular
• Be sure to input password in CAPITALS. Details are here.

• The Pay-As-You Go offer is described here, but information on the MediaNet options is scarce.

• All add-ons seem to be for mobile browsing only, but the phone can be used as a modem for a notebook (last checked June 2008). iPhone tethered to laptop with the 100MB plan and APN listed above with no issues. Only http and https were accessed though, so other protocols are unknown. (June 09)

• The ATT homepage does not mention if other services suchs as POP/SMTP, etc. are available. In practice, POP/SMTP and also IPSec for VPNs is possible (last checked June 2008).

• Network: GSM/EDGE in 1900 and 850 MHz, 3G in same bands.

• Top Up: In AT&T stores, RadioShack, 7-Eleven Stores.

• External Reports: WirelessMoves, Darla Mack