To me, data is data...The only way I can see them tracking "tethering" usage at all is app based, meaning the wifi hotspot app would have to be active for them to even know...I think their way is to embed wifi hotspot into the OS to force the issue...I'm sure someone will figure out how to defeat it, just like everything else....I'll stick to FROYO till then.
Whats funny is that I was on a leaked GB and tethering worked, then one day it stopped and re-directed me to the vzw site...So I'm back on a rooted froyo and have no problems tethering...For me this isnt something I use often, maybe when I'm traveling and dont have free wifi or if I'm someplace and need to use my wifi xoom for something...I def dont take advantage like some people do.