Customers only need to provide their phone's IMEI to AT&T online or in a store, and then they wait for Apple to unlock the phone. This wait period may be as short as a few minutes or as long as four to six days. Most customers are being told to expect to wait up to 72 hours for the unlock to go through Apple's system.
Keep in mind that only iPhones that are out of contract are eligible for an unlock; alternatively, the customer can pay out the contract kill fee and then request the unlock. Phones that are still covered by a two-year AT&T agreement are not eligible.