Microsoft has stated that USB tethering won't be available on early Windows Phone 7 devices, but that may not be the case with the Samsung Focus and Samsung Omnia 7 devices recently discovered to have a hack for USB Tethering.
While the hack hasn't been discovered to work on any other WP7 phones at this time, you can give a quick diagnostics menu option a try for yourself (but we offer no guarantees).
First, go to the phone's dialer application then type in ##634## (that will take you into the diagnostics menu), Next you need to type in *#7284# in the menu that displays. Then scroll through the phone options to "Modem, Tethered Call" where you will follow the on-screen instructions until you find the USB tethering option. If the feature is available you can simply turn it on and there you have it, USB tethering
The option isn't working for all people and definitely not on all Windows Phone 7 devices, but it's definitely worth a try. [Via PhoneArena]