A Blog entry claims that if you complain about EDGE service, AT&T will let you cancel the "required" $20 data plan on the iPhone.
Under this cheaper plan you still get 450 anytime minutes, 5000 night/weekend minutes, free mobile-to-mobile, rollover minutes, you can still use the internet features over WiFi and you dont have to pay an extra $20 a month for the privilege of enduring worse than dial-up speeds over AT&Ts Edge service.
Your mileage may vary, but there had been rumors prior to the iPhone launch that the data plan could be canceled after the fact.