Apparently, starting this Sunday, AT&T will begin to unlock non contract iPhones. Customers with iPhones with accounts in good standing will be able to request a carrier unlock for their device. What this means is that once freed from the AT&T network, any micro SIM card can be inserted into the phone.
AT&T’s official 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.