AT&T confirmed today that it will unlock customers' post-contract iPhones starting Sunday, April 8. AT&T told Engadget,
Beginning Sunday, April 8, we will offer qualifying customers the ability to unlock their AT&T iPhones. The only requirements are that a customer's account must be in good standing, their device cannot be associated with a current and active term commitment on an AT&T customer account, and they need to have fulfilled their contract term, upgraded under one of our upgrade policies or paid an early termination fee.
This is good news for customers who travel abroad. When they reach their destination, customers will be able to purchase a less-expensive SIM from a local carrier, instead of subscribing to an AT&T international plan.
We've asked AT&T to clarify if this will be only for the iPhone 4S, or other models as well, but at this time haven't received a reply.