Verizon really has no way of knowing right off if you're tethering or not, since the data packets are all 3G. HOWEVER: if you start torrenting, streaming movies all month or generally exceeding 5GB a month continuously, they may start paying more attention to you and figure out what you're doing. I tether very conservatively, and only when I need to.
Unlimited data really IS unlimited data... but keep it reasonable and you should be okay.