Have you tried it & are you saying it doesn't work? I never thought to try it on my Ultra, if I get time I will give it a go tomorrow night. On some cards you need to use the Apple DVI/TV G5 convertor, I have 1 of these so I will give it a go tomorrow night.
After much fiddling around, plug the adapter into a DVI socket & plug the lead into the onboard S video socket that is next to the DVI socket. Although you will get no video out of the adapter, the card needs to see the adapter to output video to the graphics onboard socket.
Hope that is as clear as mud! Good news though that the 6800 does video! You can either mirror the displays or have an extended desktop.