Here AT&T's statement:
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 excellent news for the many U.S. customers with locked iPhones. Perhaps emails like this to Tim Cook prompted the change in policy.