I've got a Sprint unlimited data plan with which I'd have to pay extra for tethering if I didn't use
PDANet to tether my winmo photo to my netbook. Apparently it allows my phone to dial for data when tethered in exactly the same manner that it would if I were browsing the web via my phone.
I don't have the technical knowledge to substantiate any of this but I've been using it for months and have yet to be charged for tethering.