Google news for you, If you have an iPhone from AT&T and your contract has expired or about to expire. Engadget reports that AT&T will be unlocking the iPhones that finished the two year contract terms in good standing starting from Sunday April 8. Yes after unlocking you can use any carrier/SIM if you want.
AT&T also confirmed the news and gave the following official statement to endgadget.
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