Just to add to the previous points.....I upgraded my 8310 to ATT's 110 and have had no problems with it although I am with ATT. I too would recommend this version and add the following:
Back up your device with Desktop Manager
Use DM 4.6
Remove your SD card before installing the 4.5 OS on your Curve.
The only negative I experienced with 4.5 was the loss of some volume on my mp3 songs but that was resolved with mp3 gain.
My brother tethers daily with his curve on Tmobile......I'll try to find the link or directions and post back. Good Luck.
"We make a living by what we get, we make a life by what we give."
Enjoying life in Lehi, UT with my BB 9780 on ATT