I've never been able to getna straight answer out of anyone regarding whether or not sharing the iPhone Internet connection actually violates the service agreement. The wording about usage is vague. My interpretation is they have the right to kick you off their service for whatever use they deem inappropriate, but there is no current case law where they have actually sued anyone.
I talked at length with the manager of my AT&T store about this. Her posititon was that jailbreaking voids the phone warranty but not the service agreement. Of course, that conversation does nothing to protect me from losing my service nor getting charged for overuse. I just thought it was interesting. She also said that someone higher up the food chain too her they would be offering a pay as you go tethering "plan" within the year. That dashes my hopes of AT&T just turning the feature "on".
All I can say is I have used tethering and downloaded copious amounts of data on AT&T with the iPhone and Sprint with the Treo, and I haven't seen anything unusual on either bill.
These are just observations for the sake of rhetoric and NOT legal advice.