The company is upset with such procedures as this which make it easy to input the tethering settings into your iPhone.
"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.
Disconnection and option
"Under those terms we reserve the right to charge customers making modem use of their iPhone or disconnect them. If customers wish to use the Internet Tethering feature on their iPhone 3G or iPhone 3G S, we recommend taking the Bolt On which is available from Friday 19 June."