So, I didn't realise it until today, but the pc-connectivity manager lets you do exactly what's being asked for. You can connect using USB, Bluetooth PAN, or ad-hoc wlan and share an internet connection from either side. http://pc-connectivity.garage.maemo....ion/index.html I have not tried it, but the instructions look sane.
A Maemo control panel applet to help the connection setup between the device and the hostpc.