Well if it's unlocked and directly from Samsung all you should need to do is put an ATT SIM in it. GSM-based networks like ATT shouldn't need any of this nonsense of registering the phone with the carrier, that's the whole point of the SIM technology, and unlocked means it's not restricted to a particular carrier's SIM.
(There are 2 uses of "unlocked", which should not be confused: SIM unlocked and bootloader unlocked. But an unlocked bootloader is only useful to people wishing to modify the software, which no manufacturer encourages, so if a phone is sold by the manufacturer as "unlocked" it means no SIM locking).
