In the U.S., the iPhone is currently only available on AT&T. As other posters have noted, you can "jailbreak" your phone and use it on T-Mobile's 3G network--but that is the only other US network you can use your jailbroken iPhone with. In my opinion, Jailbreaking the iPhone is risky because it voids your warranty. It is possible that Apple's exclusivity with AT&T will come to an end in the near future, but that means Apple will have to come out with a different version of the iPhone that doesn't rely upon a GSM/3G network (as I said, T-Mobile is the only other option in that category). Apple will need to put a different radio chip in the phone to make it compatible with Verizon or Sprint.