I used it quite a bit (within reason of course) on my iPhone4 and never had a problem. Also purchased Tether for iPhone while it was available and have been using it with no problems. I plan on jailbreaking my iPhone 4S when I get home from work then I'll be back to using TetherMe.
Just don't go nuts on your data usage and you should be fine.
A lot people had already posted theirs expericned on this thread. Check it out
Botton line is that nobody really know exactly how they are tracking this, but it seen that if you had an average low usage and all of the sudden you get an spike they will send you a notice (even if you are not tethering, some people had got it from just using more Hulu and netflix stuff)
I had tetherme for ip4 before and it worked perfectly which I only use occasionally. now I installed it fo the ip4s and it doesn't seem to work. has anyone gotten tetherme working on their ip4s? just making sure whether or not its compatible.