Currently Being ModeratedDec 7, 2012 4:04 AM (in response to JAMESKAO)
I would expect to see that message on a phone that was originally sold for either Verizon or Sprint, hacked to run on AT&T and then, after the iOS was updated, reverted to its original, unlocked state. Did you actually buy your phone directly from AT&T or Apple?
Currently Being ModeratedDec 10, 2012 1:18 AM (in response to JAMESKAO)
Check the model number on the back of the phone (in very hard to read type)
The AT&T iPhone 5 should be A1428
A Verizon iPhone 5 would be A1429 and will work with AT&T SIMs but cannot use North American LTE.
It's possible that they gave you a Verizon model by mistake.