I use VZW and purchased WMWifirouter like 2 months ago and love it. I haven't seen any charges, in fact I actually spoke with customer service last month before my bill hit and cleverly disguised the fact that I wanted to ensure no charges would hit. After a month of using WMWifirouter and downloading tons of crap, she looked at my bill and confirmed that she saw no tethering data at all. So I can confirm that it does not charge you for tethering. Just don't tell them you're tethering, don't run up a ridiculous amount of data. IE don't torrent off of it. I hit about 7 GB of data last month and had no additional charges.
As a side note, I've been torrenting *on* the phone using WMTorrent, and racked up about 20GB of data usage this month, without tethering. Kinda curious what will happen
