Don't know about Vista, but in XP all you need to do is go to 3G Connection Properties, Advanced Tab and click the ICS "Allow other network users to connect through this computer's internet connection".
Bridging will link the two networks, e.g. connects a wireless network to a wired one, so that packets from either will go to the other one(s).
ICS will only allow connections out in one direction, e.g. from the wireless network to go out to the internet using NAT.
I reckon we might be onto a winner here - i just enabled it so ill have a play tommorow and see what i come up with, I would try tonight but theres been more than ample red wine under the belt so best leave it alone!