On Verizon, unlimited data is called VCAST and is a $15/month charge.

Every provider, with the exception of T-Mobile, has a separate tier for Smartphones, since they expect higher data usage. Usually it's nearly $20/month for unlimited data on a "phone" and $40/month for unlimited smartphone data.

These all, of course, just like the iPhone, require an existing cellular contract for voice.

The only revelation here is that the iPhone isn't being considered as a smartphone. There are rather good reasons for this: it has no 3G. Reviews have called the cellular internet "painfully slow" (NYTimes) and have otherwise lead me to believe it's just as unusable as I'd expect!