While internet to USB tethering is not officially supported by Windows Phone 7 until the promised Q1 2011 update, it seems that devices and OS already support it. Probably Microsoft has not yet reached an agreement with some carriers and their idiotic pricing plans, so the feature is just hidden.
The good news is that if you own a Samsung or an LG you can already enable it without waiting for the update: just follow these direction. The only problem is that those settings apply to Orange France.
Being Swiss and having a Swisscom subscription (thankfully they allow tethering without any restriction) I had to play a bit but in the end I found out that these settings work fine with Swisscom:
Modem configuration: +cgdcont=1,”IP”,”gprs.swisscom.ch”
Call number: *99#***1# (instead of *99#)
Enjoy!