If you fall under the list of requirements set forth by AT&T, you and your iPhone will have access to micro SIMs that are compatible with the devices radio bands, and thus allowing you to use it on different networks. According to the statement sent to MacRumors:
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 will be the very first oppurtunity for the iPhone to see official unlocks, rather than through hacks. About time! Via: MacRumors