Well, I would never say never.... but... here's my own personal experience with it:
I've been using it for a few weeks now, and just received my VerizonWireless bill. I opened the PDF of my bill, and since my number on the account is the main number, I was a little scared that I was going to be charged or there was going to be something within the bill (such as a order to stop using an illegal app or something.....).
At any rate, I used about 1GB of data using TetherBerry, and guess what??
Absolutely no charges whatsoever for data usage from VerizonWireless!!
So, I am a very happy camper thus far ;-)
BUT like you say, you never know.... but it appears that the folks over at TetherBerry know a little about what they are talking about, or VZ would have certainly hit me up for tethering without having a actual tether account with them.
Just my two cents,
Statustray

