Tethering is availble only if your on the 5GB or 10GB data plan or if you have the 2GB data plan with the 15 dollar hotspot feature! If your on the recently released truly unlimited data feature t-mobile blocks the tethering!
Weird thing when I first got my 810 is that tethering option in settings didn't appear until I factory reset my phone about a week later!
Ok this will be interesting! I have the $25 mobile hotspot plan; 1.5GB data only (skype and google voice setup) Been doing this with galaxy nexus 4-5 months now. I'm kind of surprised how often I use the hotspot feature.
I'm gonna have to go to the Tmo store anyway to get a microsim, hopefully they can help me out too.
Also, maybe everyone already knows this (I didn't realize it though) that if you have WIFI turned on, mobile hotspot won't work, at least as long as it's connected to a wifi signal. Of course you would not need it in that situation, but I'm just sayin' - it's worth checking if you think it should work and it's not working for you.