I'm on t-mobile too and i noticed this feature has stopped working (on my desire) even though it has worked in the past. All I can assume is that t-mobile have blocked this service and now only offer it with their (more expensive) web n walk max plan - the same plan you need to have to be able to make skype and voip calls over 3G.
I doubt its a kernel issue because it worked fine when i was on holiday in India where I purchased a 3g sim card with an unlimited data plan.
Just out of curiosity i tried the wifi hotspot again and to my surprise it now works. I didn't change any of the settings on my laptop or the phone. So i guess the service isnt just available on webnwalk max. I don't know what causes the problem - sorry.