An unlocked iPhone 4S is supposed to 'officially' come out in November but it looks like we don't have to wait that long. According to 9to5Mac, if you bought an off-contract iPhone 4S from AT&T, your phone is already unlocked.
9to5Mac says all the user needs to do is purchase the iPhone 4S off-contract and then plug the phone into iTunes. Once plugged in, that pop-up from above will appear and you can use the iPhone 4S on any GSM network (namely T-Mobile or international flavors when you travel). Not a bad deal! The current off-contract prices for the iPhone 4S are $649 for the 16GB model, $749 for the 32GB and $849 for the 64GB. The same cost as the 'official' unlocked version that's coming in November. [9to5Mac]