I've heard that the belgian iPhones, while "unlocked", only work with Belgian cell phone companies. I don't have any source, but its something to consider and watch out for. Somebody said its called "Nation Locking" or something.
I figured there would be all kind of tricks up AppleT&T's sleeves this time and if you want an unlocked phone, it's probably worth waiting a few months (weeks?) until the proper jailbreaking procedure is figured out.