What should be offensive is having to pay for tethering at all. You're paying for... Wait for it.... Nothing. Which is another way of saying what you've described, ATT doesn't actually want you to use 2GB or even 200MB when you're paying for those plans.

Tethering on the iPhone has been around and fully functional for almost a year. There might be something new in OS 4, but you don't have to wait to find out how tethering itself works, you can go read about it now. It uses either the USB dock connector or Bluetooth. I think it would be trivial for them to extend this to using WiFi and the reason it's not there already was probably to have something to release later or to appease carriers who don't want you setting up little private 3G networks.