I think recently T Mobile has changed something. I am no longer able to tether using my Nexus. It started last week, I've found a couple of workaround, first using PDA net I set it to hide use from carrier I can't think of the exact wording but it's something like that. Also wireless tethering doesn't work only USB has been successful for me. Second, if you change your user agent in you browser to something other than a desktop browser it will work.
These two options have been the only way to tether ever since I've noticed the problem.