As of April 8, AT&T is unlocking iPhones. Now, it won't unlock just any iPhone; there are stipulations you'll have to meet before being freed from the carrier.
The requirements, according to a statement issued to CNET, 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."
If your account and device fall within those guidelines, you can request an unlock. According to The Next Web, it's a fairly painless process.