It seems that most major Ubuntu updates recently have borked my X server config in some way. One of the problems that took me the longest to track down was related to suspend/resume and the fact that the monitor never woke up when the laptop lid was used to initiate the suspend operation.

If you suspect X server problems you can execute

dpkg-reconfigure -phigh xserver-xorg 

to automatically reconfigure your xorg.conf file (this will lose any manual changes you may have made to the file, although a backup will be kept and stored in /etc/X11/.

From Ubuntu 8.04 onwards xorg.conf appears to be in the process of being phased out anyway, so manual tweaking should be unnecessary before too long.