If you believe that your AT&T iPhone 5, which you paid full price for and is off contract is already unlocked when you get it, then think again.
AT&T iPhone 5 easily unlocked by doing restore on iTunes
Verizon offer the iPhone 5 and it does happen to be unlocked when you get it. However if you get the device with AT&T and you pay $649 instead of taking out a contract over two years, you have to go through a long procedure to unlock your handset.
Customers would generally have to fill in a form online or send out a fax to AT&T and then wait to find out if they will unlock your iPhone 5. However it seems that iPhones from AT&T can easily be unlocked just by restoring the iPhone 5 in iTunes (hook your iPhone 5 up to your computer and launch iTunes then restore it just how you would when doing a factory reset).
TechCrunch were the first to find out about the bypass and the process was confirmed with technical support from AT&T. The iPhone 5 was reset and then used on a T-Mobile SIM, with the user receiving the message congratulating them on unlocking their iPhone.
You can confirm whether you successfully unlocked your iPhone 5 by putting in a GSM SIM card. You can even cut a micro-SIM into the size of the new nano SIM and use that. In seconds the phone should pick up the new network and all your calls and data then work as you would expect.
CNET have not actually tested this out but an iTunes reset is needed for the approved AT&T unlock too. Why AT&T would make their customers go through the hassle of posting a request to unlock the phone is not known, if you can perform the process yourself and AT&T could tell consumers about the iTunes process online.