I recently stumbled onto the Internet Connection Sharing feature of NetworkManager ("Shared to other computers") and it worked pretty okay. But it doesn't seem to support UPnP port-forwarding for systems on the shared connection. (My xbox 360 reported that there was a "moderate NAT", the error it reports when it can't forward it's ports, restricting who it can connect to.)

NetworkManager seems to be aiming for everything to "Just work", and I figure this would majorly increase it's usefulness if it could forward ports to client systems automatically.