(UPDATE: The earlier claims (see below) have since been debunked as innacurate.
Joseph Brown, a developer who's posted hacked carrier updates for AT&T, Sprint, Verizon, and T-Mobile, has posted a lengthy blog post charging that “Apple limits devices to even out” the networks of its carrier partners.
Brown's theory is that Apple limits the capabilities of the device in order to combat the large amount of data/bandwidth iOS device users consume and ease congestion on carrier networks. Brown found signs of throttling data speeds for Verizon and Sprint too.
The only carrier he says is apparently not limiting the iPhone 5′s capabilities is T-Mobile.