Idk how much you get charged but i haven't really seen anyone get caught yet. I dont really know how they would find out either. But i think you need to ROOT etc...to tether.
Anyways, check the coverage etc...and see if any of them are bad in your area. I have T-mobile currently and it's been fine for me. I live in New York so all four carriers are good here.
You definitely will save some money going with T-mobile and it's not that bad.......just check the coverage map to see how 3G data and Voice coverage is in your area.
Plus, you can just get the Nexus One already for T-mobile or the HD2. Your out of luck though for most unlocked phones, T-mobile uses a frequency most unlocked phones don't use for 3G data. AT&T uses the more common 3G data frequency if im not mistaken. AT&T also has the fastest 3G speeds right now, proven by engadget themselves recently, few months ago.