Why do so many people think these plans will be so expensive? Looking at Cingular's current plans, unlimited data goes for $20/month and 450 minutes of voice goes for $40/month. Even at 900 minutes, the most you'll be paying for is $80/month.
In terms of web browsing, the iPhone doesn't use more data, it simply displays it in its true form. In turn, I guess that could lead to more demand for bandwidth since using the web on the phone will be so much easier, but for all intents and purposes, it's no different than a smart phone.
For those of you who say we need it for tethering purposes (i.e. using the phone as a modem...much more expensive), I think that will be pretty rare. You can already get email, calendar, contacts, and internet on the iPhone...using the iPhone as a modem for your laptop will be pretty rare to start, I think.
Other thoughts?