I don't think there is a chance your plan will be terminated. I think at&t will send you a warning text first, and then just switch you automatically to a tethering plan (in effect killing your unlimited data) on subsequent offenses.
Some people tether and don't get these text messages.
If enough people do this, then we can see a broader reaction. I needed to tether, so I got the 4GB plan (it's now 5) on my iPhone and tether using the built in iOS tethering feature. Even if AT&T comes out with a tethering option on the iPad I won't get it, because it will most certainly mean giving up my unlimited plan, and I'd rather not do that.
Sure, I could tether regardless, but I have a peculiar habit of abiding by the terms of service to the best of my ability. It's just a personality quirk of mine. Ignore me.