I have a rooted Samsung Precedent (Walmart Straight Talk pre-paid - unlimited $45.00 + tax per month) with Android Tether but it stopped working. Since the friend who rooted the phone for me and installed Android Tether is no longer available to help fix the often quirky Android Tether I tried "Fox Fi".
It worked perfectly (and since it doesn't require root, no superuser permission needed)!
I'm sold. Installed it, rebooted (to get a fresh start) and it worked perfectly and can not be detected by your service provider!
There are some phone it doesn't work with so check the description in Google Play (market)!
AND its free!
Well worth a try (especially if you uncomfortable rooting your phone).