It has been said that the estimated data usage amount is 5GB/month before Verizon starts monitoring your account. I have not heard of anyone being caught or penalized, but it is clearly stated in the TOU that you signed when you signed your agreement that you cannot tether your phones data plan to anything without explicit consent from Verizon. I assume that the only way you will ever get explicit consent is to purchase a tethering package. Those are $20 for wired tethering. I don't think they would approve of wireless (WIFI) tethering because the only way you can do that is to root your phone and that voids your warranty.
Now, with that being said, I sometimes have to tether for school. This is usually when I am at my girlfriends house as she doesn't have internet. I probably use a max of 10-20 MB/ month by tethering. I usually only use a total of 2-2.5 GB/month of data in general. I think as long as you are smart about it and don't start downloading HD movies or using torrents while tethering, you should be just fine.
(Disclaimer: I take no responsibility if you get caught. I am in no way telling you that it is OK to tether or that you will not be penalized. My statements are of my own observations and in no way are guaranteeing that you will not get caught.)
Kratos