AT&T phones will not work with other US carriers
Unless I am mistaken, I think that the US iPhone only used AT&T's GSM bands, so unlocking would not mean that you could use on any of the other US GSM carriers (eg T-Mobile).
CDMA is completely different as well, so having your phone unlocked would be pointless unless you go overseas (and even then it might not work).
The exclusivity deal was really a technology lock in. The exclusive deal was only Apple not creating a CDMA device, or a multiband GSM radio.
Not that I want to stick up for AT&T and Apple, but I can't really see where the problem is.....