Sorry but not all service areas are like yours. I enjoy great speeds and coverage and therefor think AT&T is great; however, I don't make mindless posts saying "AT&T is Awesome". Vice-versa can be said in your case.
In any case, yes AT&T can know when you tether (via 3.0.0's solution) because the iPhone sends/receives data packets to AT&T that contain identifiers as if the iPhone is a modem/router for another device (in this case your computer).
The jailbreak method doesn't have identifiers. That is because the tethering hack via jailbreak uses SSH. The iPhone sends/receives packets to AT&T as if it alone were using the Internet. However, on a back door the iPhone is using a secret & secure connection (Wi-Fi usually) to relay/retransmit that data to another device (your computer).