AT&T can do it for you, it's whether or not they WILL. as long as the phone is paid for (from AT&T) you might be able to get them to unlock it.
try calling them, you may have to try a few times to find someone who will do it... if there is a shop locally, you can try the service department there... being face to face with someone can be more sucessful.
OR you can try one of the many online unlocking services... but it's a real crap shoot.