The same is true of minute bundles. Users which go a lot over (and pay per minute) are fine, users which are a lot under (and still pay for the bundle) are fine. Users who are averaging use of 80-120% will probably be costing the provider more than they pay.
On Sprint and T-Mobile, yeah, we have unlimited data plans.
If Verizon were to offer the iPhone, a lot of the AT&T users with unlimited plans who are using a ton of bandwidth would jump to Verizon, and, if they were to try to return to AT&T, wouldn't be able to obtain an unlimited plan.