First things first, if you use multiple monitors and you're thinking about upgrading to Ubuntu 8.10 (Intrepid), I would recommend holding off. A good number of users, myself included, have experienced a very annoying bug where the mouse spontaneously loses it's ability to click. The only way to regain mouse support is to kill X with ctrl-alt-backspace (ugh). I've been able to faithfully reproduce the bug by doing the following:
* Enable Xinerama in xorg.conf
* Open firefox fullscreen on both monitors
* Rapidly move the mouse between monitors
It's worth noting that this bug has nothing to do with what window manager you're using. I've experienced it under Gnome, AwesomeWM, and Xmonad. The bad news is that even though a lot of people are complaining about it, the bug has not been fixed. As of right now, the only reliable method to regain workstation stability seems to be downgrading X.org back to the Hardy packages. To assist anyone who might be Google'ing this issue, here are the instructions for doing so.
Log into a text console as root and shut down X with '/etc/init.d/gdm stop'.
Add the following to your /etc/apt/sources.list:
deb http://us.archive.ubuntu.com/ubuntu hardy main
deb http://us.archive.ubuntu.com/ubuntu hardy-updates main
deb http://us.archive.ubuntu.com/ubuntu hardy-security main
Run apt-get update.
Purge all the packages you can by doing the following:
dpkg --purge `ls *xorg*.list | sed s/.list// `
Some of these will fail to purge due to dependency issues. That's OK. Continue with the instructions.
Reinstall the packages from the Hardy repo:
apt-get --reinstall install x11-common/hardy xorg/hardy xserver-xorg/hardy xserver-xorg-core/hardy xserver-xorg-input-kbd/hardy xserver-xorg-input-mouse/hardy
Note that if you're using a stock Ubuntu video driver such as the Matrox driver you will need to include it in the list as well (xserver-xorg-video-mga/hardy). I use the third-party Nvidia driver, so it wasn't necessary to include it in the list.
After this, you can start X with "/etc/init.d/gdm start" and hopefully login to a working desktop. The final step is to go into synaptic and lock the reinstalled packages to avoid the upgrade manager from nagging.
Thanks to Nelson for providing these instructions, which I've modified slightly for presentation here.