Recent Ubuntu distributions don't use a physical /etc/X11/xorg.conf file by default - they detect everything at runtime. If you need to set some settings yourself in xorg.conf, this is problematic. To override this behaviour, you need just create a new /etc/X11/xorg.conf and this will be used instead.

To generate one based on what has been detected, stop X and then run Xorg -configure:

$ sudo /etc/init.d/gdm stop
$ sudo Xorg -configure

This will write a new xorg.conf file to your home area. This will need to be copied and renamed to /etc/X11/xorg.conf