Many people on the forums have mentioned the contract between the two is exclusive for five years. In that case, it's not tied to a particular generation of iPhone. If Apple tried to break the contract, then AT&T would've been able to sue them for breach of contract law--and rightfully so.
(I'm not a lawyer, but I did stay at a holiday inn express last night

)
Verizon and Sprint would still have to agree to Apple's terms (e.g., keeping their store off the iPhone, etc.), which they might not be willing to do. AT&T was willing to do that; hence, they got the iPhone.
Esther