I have bought three iphone 4S recently. 1) an AT&T upgraded, discount ($199) contract one (locked for sure) 2) and off contract Device only one (unlocked, still unlocked after upgrade to 5.01, and i think it will be always unlocked for sure), Both are MC918LL/a. 3) The office unlocked one I just ordered from Apple Store online is MD234LL/a. So I think the model # really does NOT matter!!! As soon as it is not locked in apple system.