I upgraded my 10 month old 7100t to 4.1 a few weeks ago and have noticed the same bluetooth headset problems (Motorola HS-820). After extensive trouble shooting T-Mobile agreed to send me a replacement 7100t.
The new 7100t now has the dreaded re-boot without warning problem whenever I turn on bluetooth! (with or without the headset paired)
I am sending the new 7100t back to T-Mo and will try it again next week when I receive the second one. Hopefully this will work!
In the meantime, I will use my old 7100t and manually pair my headset each and every time!