I received a new monitor for xmas and wanted to set up dual display (twinview) for Ubuntu 9.10. I already had the proprietary Nvidia drivers install, and it was very easy to get the dual monitors up and running. However, I always got this error when trying to save the configuration: Can't parse /etc/X11/xorg.conf. And upon restarting, only one screen would show and I would have to set it up again. Even when I launched Nvidia with sudo (sudo nvidia-settings), I still got this error on the command line:

VALIDATION ERROR: Data incomplete in file /etc/X11/xorg.conf. Undefined Device "(null)" referenced by Screen "Default Screen".

I moved /etc/X11/xorg.conf to a new filename, ran sudo nvidia-settings and was able to save as new xorg.conf file and now I have both monitors on start up. Also looks like you can simply edit the existing xorg.conf file and you will be able to save your settings.