Microsoft recently revealed that Windows Phone Mango will not support internet sharing with PCs out of the box, but unfortunately did not give a clear commitment about including it in a future update.
In the comments section on the Windows Team Blog Matt Bencke, a General Manager for Windows Phone 7 explained the reasons for the delay was that these days Microsoft had to work with carriers to enable the functionality (and presumably allow billing) unlike in the past.
I appreciate your pain around tethering (using your device as a modem) as this has been something that I’ve used in the past. Tethering, in the past was something that we managed independently of the Mobile Operators (e.g. AT&T, Orange, Verizon). In recent years, the Mobile Operators have now taken ownership of this functionality x-all platforms (e.g. our competitors as well as us). We are continually working with them and our OEM partners to enable this functionality for our devices, but at this time there isn’t an announcement that we’ve made in regards to rolling out this feature. Stay Tuned!
Bencke did not reveal the time scale for the roll-out of this feature, but I am sure carriers are as eager to get a billable feature on their phones as Microsoft are to add it.