Sorry to double post. That was an interesting article, but not 100% consistent with what I read here. I thought most of the people who got warnings from AT&T were using MyWi/PdaNet, which does not use the built-in tethering capability (though I'll admit I am not familiar with PdaNet). People using TetherMe, which uses the built-in hotsput functionality, haven't gotten warned as much by AT&T. I could be wrong though. I am thinking about switching from MyWi to TetherMe after reading the stories here.
FWIW, I used 17gb last month, maybe 100mb of that tethered with TetherMe, and nothing so far from AT&T.