An AT&T decision to implement usage based pricing for data would be akin to telling an already angry mob to “f%ck off!
AT&T already suffers from a perception that its network is lousy. The iPhone exclusivity deal has sustained the company for the last few years because the device is just that good that it has brought in additional subscribers despite AT&T’s network. The smart play for AT&T would have been use the period of exclusivity wisely by borrowing all the money they could and investing heavily in building out their network. If AT&T could get their network performance to a level nearly on par with Verizon, they might head off the coming exodus of subscribers. But they haven’t done that (or at least the perception is that they haven’t, either way the point is the same).
Given this perception, why on earth would they want to add further fuel to an already raging inferno (of unhappy subscribers) that love the iPhone, but can’t wait to dump AT&T for Verizon when the latter gets the phone. If anything, AT&T should be reducing its prices for data as a means of compelling subscribers to stick around, as some are already heading to Verizon now that it has the Droid.