said by TW :

Those of you who are bragging about getting tethering for free on your WinMo phones should check your Terms of Service and perhaps pipe down. All recent ToS say you have to buy a tethering plan to tether. A few years ago, AT&T was in sort of a don't ask, don't tell mode about tethering. Now, they have tethering plans for just about all phone except the iPhone. Just because you can do it doesn't mean you're not violating your ToS. I may not agree with AT&T's policies, but if I violate their ToS, they have the right to kick me off.