It might not have anything to do with the iPhone, either. I would think Apple will fight tooth and nail to prevent a situation where iPhone users were not allowed to upgrade to a new model, as pushing users to upgrade more frequently than strictly necessary (or common among users of similar devices) is essentially a cornerstone of Apple's way of making money. Considering that the iPhone does have some status symbol value to some people, and how quickly Apple revamps their products in comparison to the two year contract cycle many carriers use, there's just no way. It would breed a
huge amount of resentment if a really interesting new iPhone came out and existing users couldn't upgrade to it.
On the other hand, Apple could negotiate it so their terms are specially favorable -- working iPhones could be worth enough to make the iPhone trade-in worth it, and/or they could get special status so that iPhone traders could get a new iPhone at subsidy price. The tactical advantage to such a situation (and nuisance to me

) would be that this would probably significantly crimp the market for unlocked or unlockable iPhones sold used without contract. And while I definitely want to upgrade my iPhone at some point, I'm still not entirely ready to move to AT&T.... So from that standpoint, there could be strategic value in it for Apple.