The new Windows 7 phones apparantly don’t support tethering, possibly due to increased pressure from the phone companies, who want to sell Dongles on seperate plans?
Here is a way to enable it on a Samsung Omnia 7:-
1) dial ##634# on your keypad then press Call. You’ll go into the Diagnosis Menu with a number keypad.
2) enter *#7284# and a Micro USB Test dialogue will appear, make sure you have setup Zune with wireless sync first, otherwise Zune stops working.
3) find out the dial-up connection settings with your carrier. and create a “dial-up” modem on your PC, It will usually be something like this:-
Dial number: *99***#