Not possible to 'unlock' an AT&T iphone as they refuse to have them taken of their networks, the only option is to 'jailbreak' the phone, and places like wan chai computer centre or 298, or golden computer arcade in sham shui Po, but you will have to do this every time you update the software, and depending on where you go it costs anywhere between 100 and 500 dollars.

Sorry just googled it to check, AT & T have changed their mind, and will now unlock for you, it is all done remotely, as is actually legit, whilst jail breaking isnt!

thank you