Skull's right, j0sh is just paranoid or misinformed. MyWi is detectable because the signal is routed through a 3rd party application so the data has an app signature that the carriers can look for. TetherMe enables native tethering so to the carriers, the data that is being tethered appears as if it were just normal data being used by the device itself. The only thing(within reason) that they could do, would be to assume that you are tethering based on the amount of bandwidth you are using, but that would never justify them being able to do something about it.
If it makes you feel any better, I have been tethering with TetherMe on Verizon since November with an average of 2-3gb/month in tethering alone and I have yet to receive so much as a warning.