It is too bad there's not a Jailbreak FAQ on MR. (maybe there is and I missed it).
This has to be one of the most asked questions.
Q: Have people without a tethering plan been caught by their carrier?
A: Yes. However, it was years ago when ATT had an unlimited data plan and their tethering plan was stupidly expensive.
Q: Do the various jb tethering apps block or hide tethering really work?
A: The only app that claim this, AFAIK, is PDANet. However, the lack of reports from a PDANet user getting caught doesn't prove anything since you can't prove a negative, e.g., I used TetherMe and used xx gigs of data and wasn't caught. Proves nothing.
Q: Can tethering really be masked?
A: No. If the carrier really wants to dig into their network logs, they can find you. The question is not can they find you, the question is "Is it worth the trouble and cost to find you?"
Q: Do we know what criteria carriers would use to find you?
A: No.
Q: Does not using more data than you typically use mean you wont get caught?
A: No.
Q: Then what will ensure I don't get caught?
A: Buying a tethering plan.
Q: Will I go crazy if I have to track all my tethering usage?
A: Yes.
Q: Is the argument that data is data and carriers should not distinguish how you use data a new argument?
A: No.
Q: Then why do carriers distinguish data from your phone and data from tethering and charge for it?
A: Because they can. Get over it.
Q: Does complaining on a discussion forum help?
A: No.
Q: What can we do about it?
A: Switch carriers.
Q: Are there any new arguments about tethering and carriers?
A: No.
My Conclusions
1. The carriers have addressed this in other ways. They've lowered the price of their tethering plans, throttling old unlimited plans when certain data thresholds are reached are just two examples.
2. Before asking questions related to tethering they need to ask themselves a simple question:
Q: What's the worst that can happen?
A: The carrier discovers it and sends you an email/letter to stop or buy a tethering plan.
Even ATT gave people a warning before automatically adding the tethering option to someones account and this was years ago.