In addition, the story goes that many changes needed to be made on the carrier's side to support features like visual voice mail. Plus, Apple wanted control of the user experience with iTunes, and didn't want buying an iPhone to be like buying a typical cell phone. Supposedly, AT&T was the only company to agree to all this and take a chance on a new, unproven product.
So, they get props for at least being a bit innovative. It would be great, though, if the iPhone were available to anyone on any carrier.