This is obviously a challenging question for us to answer definitively, but this is what we do know.
Carriers typically route their tethered data traffic across specific APNs. Therefore, if you bought the tethering service through a carrier, your tethered data would use these APNs. Based on user stories online, it appears that some users who have "jail broken" their phones have had their tethering activity detected by some carriers since many of the jailbroken tethering solutions use these same APNs and are thus easily detected.
Unlike many of the "jailbroken" tethering solutions, our service does NOT run across carrier tethering APNs.
Tether.com has been around for 3 years with more than 500K downloads around the world. Currently, we are unaware of any user having an issue with their carrier based solely on the use of our product. Our product can consume significant amounts of data on your existing data plan, so we do encourage users to ensure that they have an appropriate sized data plan prior to using Tether.com's products. If you have concerns, we also recommend that you review your own carrier TOS before purchasing our product.