Speaking as an employee of T-mobile, I am seriously confused as to how the company is going to implement these charges. I just got the mytouch 4g as a champion device, and the tethering works just fine for me! I didn't change my account or anything, but then again I am not on a standard account. Now, I doubt that the phone automatically disables the feature when it detects an account without the tethering feature, but maybe T-Mo has something up their sleeve. They like to leave us in the dark with the important details.
Edit: Also didn't consider that maybe it's something to do with the champion device itself. Perhaps they left it unlocked for us to screw with, but I'll test a retail unit soon and see if there's a difference.