afaik, I don't think official support for tethering is on the 3G phone either. Maybe someone will hack it in later? Or maybe not if they included a "counter" to that in the new device.
Can you tether the original through WiFi?
I know Windows Mobile devices can tether via WiFi with an app that makes the phone a "mobile hotspot" and you can even put WEP, WPA on it, etc. or just leave it open for the public. But Windows has this thing called "Internet Connection Sharing" built into the OS. The funny thing about this is that you don't even need a tethering plan because it just uses the data plan. heh
As far as tethering boosting sales...it would mostly be for the business people...which would take a while before businesses adopt iPhones at the rate of other phones. For now, iPhones are still considered "toys" in that respect.