Dan, the way it works (on Sprint and Verizon) is that when you go to use their App (Verizon Mifi, Sprint Hotspot) it will phone home and register with your account and upgrade it automatically. If it's a 3rd party tethering app there's really no way that they can know its going on. I prefer to use Wireless Tether for Root Users
since I'm rooted. It has a few limitations though like not being able to support infrastructure (wireless AP vs ad-hoc) mode. But those limitations are usually per-phone and yours might not be that way.
I recently upgraded to an unofficial 2.2 leak for my epic along with a nice gingerbread theme... I'm really digging the look and feel of this. very responsive GPS now. Not to mention it looks just like a Nexus S except now I'm rocking a keyboard