Where is the X.org config file? How do I configure X there?

The xorg.conf does not exist by default any more. You CAN create one though.

Boot into recovery mode and select Root Shell. Then run:

X -configure

Then:

cp /root/xorg.conf.new /etc/X11/xorg.conf

Reboot and you can edit the new Xorg.conf.

The configurations files are at /usr/share/X11/xorg.conf.d

They are:

10-evdev.conf

11-evdev-quirks.conf

50-vmmouse.conf

50-wacom.conf

51-synaptics-quirks.conf

Check the current manual.

If you create a xorg.conf file the configurations of this file will prevail.
Also check this answer.

Usually, you don't need the xorg.conf any more.

If you need to configure some devices anyway, you can do so by placing a file in the /usr/lib/X11/xorg.conf.d/ (Ubuntu 10.04) or /usr/share/X11/xorg.conf.d/ (since Ubuntu 10.10). There are some files in this directory already.

You can find more information on xorg.conf.d (in german, but the configuration files are in english of course). What is important is that the filenames should start with a two-digit number greater than 10.

Another guide - in english - is on x org archive. It's still using /usr/lib but it's good.