Don’t waste time manually fixing xorg.conf like I did. Trying to be clever, and set my laptop up to output to both it’s builtin screen and an external monitor, I managed to break X. Trying to be even cleverer, I tried to manually fix the small mess that Gutsy’s screen administration tool had left in my conf file. Then I decided to take the advice at the top of the file:
sudo dpkg-reconfigure -phigh xserver-xorg
Which fixed everything. Hooray!