Deactivate your phone with Redsn0w for AT&T iPhone unlocking
Since Easter, AT&T has been unlocking off-contract iPhones for use on other carriers. Users who wish to take advantage of their new policy, but have previously hacktivated their phone in order to perform a software unlock. In order for the AT&T iPhone unlock to work, you must first deactivate your device. One way to do this is to restore and rejailbreak, but luckily for those who do not want to have to restore, Redsn0w can deactivate the phone for you.
Download the latest version of Redsn0w and on the main screen select "Jailbreak." Put your device into DFU mode and let Redsn0w apply the limera1n exploit to your device. Once limera1n has been initialized you will get an options screen. On this screen, make sure to only select the "deactivate" option, uncheck all other boxes.
Once Redsn0w has completed, you can remove your SIM and insert the SIM from your unofficial carrier. The unlock should now work without the use of ultrasn0w.