The iPhone is supposed to require an AT&T contract to work. So, in theory, you can't pay $600 and just use it. Unless you cancel your AT&T service within 3 days of signing up - (alexking.org). For now the iPhone keeps working. This will be more interesting if/when Apple adds Skype/VOIP support and allows one to use it as an external drive.
Apple's contract with AT&T may require them to disable this trick, and one assumes they get money from the AT&T monthly fee too. So it wouldn't be surprising if a future update blocks it. If Apple doesn't block it, that would say something about where they want the iPhone to go. (Via Daring Fireball)