I think it's a great idea, and they're going about it in a very fair way. At least they give you ample warning before upgrading you, and I've heard it's quite easy to dispute it if they do contact you. They're not being Nazis about it, and that's the important part. Unauthorized tethering is now technically stealing, since AT&T offers plans for it. It's circumventing those plans. Before AT&T offered tethering plans, it was okay to do this jailbroken tethering, but when they introduced tethering I think that kind of pushed those jailbroken programs more into the "illegal" realm since you were circumventing the plans, no matter your original intention. AT&T is being surprisingly nice about it.
The whole point of the tethering in the first place was basically to cheat the system, it'd be naive not to expect the system to fight back.
