One of the iPhone hackers (developer of course) claims that he found a way to unlock the iPhone 4S without any hardware or software changes. This unlock seems to be working (I haven’t tried yet with my AT&T iPhone 4S) only with AT&T version only.
According to Chronic, this procedure will assist you to use your iPhone 4S on T-mobile’s network with T-mobile SIM card. You will need to cut the T-mobile SIM to fit into micro SIM slot of iPhone 4S. Proceed at your own risk –
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
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
Above procedure is as described on Pastebin. Come back again to this post for more updates on this. Meanwhile, if anybody tried this procedure on your devices, let us know in the comments section below.