From what the OP is saying, it sounds like he is in on T-mobile US.

For what it is worth, I am also a T-mobile US customer. I have not had any problems tethering (via USB) a N900 with both a windows computer utilizing OVI Suite and a laptop with Ubuntu. However, I have a grandfathered "Total Internet" plan that does not explicitly disallow tethering.

To the OP: what APN are you using? Switching the APN to internet2.voicestream.com might help your situation...