Just read this article over at Techrader They'll disconnect or charge people who they think are tethering for free. Still not sure how they would know other than an increase in usage. "Internet rumours suggest that some customers have modified their iPhone to enable Internet Tethering without the purchase of the Internet Tethering Bolt On. Any use of this particular feature without the purchase of the Bolt on is specifically prohibited under our terms of service... ...Under those terms we reserve the right to charge customers making modem use of their iPhone or disconnect them." You have been warned!