Chronic has tweeted a loophole to unlock an AT&T iPhone 4S without a jailbreak or any sort of hardware modifications. It requires a cut-down T-Mobile SIM or microSIM, but other than that, the process seems simple enough.
Proceed at your own risk, we have not tested this ourselves. Initial reports are very mixed, so be wary. But, if it does work, make sure to let us know in the comments!
Insert original carrier AT&T SIM card
Dial 611 for AT&T customer service hotline and drop the call
Turn on Airplane Mode
Take out AT&T SIM card
Insert T-Mobile SIM card
Read on for the full instructions!
Make sure WiFi is off ( also tap on ‘Forget this Network” to make sure it doesn’t connect automatically later)
Switch off Airplane Mode and iPhone will search for network. This is followed by the Apple splash screen appearing.
Activation Required will be displayed on the screen
EDGE network will activate automatically – notice the ‘E’ on the top left corner of the screen
Wait for about 20-30 seconds and turn off the phone
Turn on iPhone and the same Activation Required screen will be displayed
When you see one signal bar, tap on Use Cellular Connection
Eject SIM card
Activation Required screen will be displayed the second time
Insert SIM card