I'd be almost certain that this was discussed when renewing their exclusivity deal.
However, your phrasing implies that you think Apple is "stuck" and getting shafted by AT&T. Really, it is quite the opposite. Apple surely looooves the exclusivity deal because they can leverage it over AT&T to collect globs while AT&T eats the cost over the life of the subscription. The exclusivity deal is one of the key reasons that Apple is able to offer such fantastic hardware at their current price point.
Are you kidding me? It's AT and fucking T. They don't let you get out of your two-year contract without exacting their poiund of flesh; what makes you think they'll let something as lucrative as iPhone exclusivity just wriggle away?
It's not a let them sort of thing. It could be violating some portion of the "marketability of the device" or any other sort of wiggle room words good lawyers can use to make raw deals grounds for cancelling a contract.
However, your phrasing implies that you think Apple is "stuck" and getting shafted by AT&T. Really, it is quite the opposite. Apple surely looooves the exclusivity deal because they can leverage it over AT&T to collect globs while AT&T eats the cost over the life of the subscription. The exclusivity deal is one of the key reasons that Apple is able to offer such fantastic hardware at their current price point.