I'm betting it'll be on 3 of 4 major carriers, probably AT&T, Verizon, and T-Mobile. Using Microsoft's vast amounts of money, it'll probably have its own fancy iphone-like display to set it apart.
Edit: I don't dislike Sprint, but I just have a funny feeling they won't be on board with it without some ridiculous deal. The 2.0 version will be on Sprint.