The likelihood of Apple giving Sprint an exclusive to their newest device this late in the game (ie, they're not an unknown quantity like they were in 2007) is highly unlikely.

Where one to entertain this crazy notion though the reasoning could center around Sprint's WiMAX deployment and the iPhone 5's need for higher bandwidth. Should the much rumored "Assistant" feature of the new iPhone require a bigger pipe to the Internet to function, then this could be plausible.

Yet in the end, why? Apple has only expanded their supported carriers since 2007. Centering on Sprint in the hope of forcing the hand of AT&T and Verizon in the US is a fools errand. They would be effectively shrinking the market they worked so hard to expand into. Seems nuts.