The question was for ANY Iphone model.
Since the 4s was just released it should be a least 2 yrs from date of purchase before AT&T has to unlock it.
Before the smart phone craze, a law was past that a cell phone provider had to provide the subsidy unlock for a customer after a certain length of time. I believe it was 90 days. I have other non-smart phones unlocked but I have yet to call up a try to get any Iphones unlocked. I seem to remember reading somewhere that after your contract expired AT&T would provide the subsidy unlock code.