yeah, whether usb tethering via froyo or pdanet or wireless tether for root, it's more or less the same end game. if verizon can detect the difference in the packets being sent over the network, they can detect it. I wouldn't worry about it, but as it goes,these things should not be abused imo.
In terms of 5GB limit, for me at least, this is an artificial number, as my plan is definitely unlimited. When I upgraded my contract I specifically asked whether there were caveats attached (i.e. its unlimited but capped at 5GB what???) and they said no, its unlimited. However, tethering is considered a feature/upgrade and often carriers will charge extra solely to use this. Verizon did have plans to offer tethering for an extra 30 bucks a month for 5GB, but this was exclusively for the tethering addition and WHEN it became available. This had nothing to do with your 30 bucks a month unlimited data... Sounds contradictory i know, but thats what they told me. All this said, this was back in nov when I first got my droid, so who knows now.