If you’re running Ubuntu, you might get an update notification for xorg-core. I’d advise not installing it. It broke Xorg on my computer and this seems to be a common problem.
The solution posted on the Ubuntu Forums is to downgrade the package:
sudo apt-get install xserver-xorg-core=1:1.0.2-0ubuntu10
And then restarting X. It’s strange that an update to a core element resulted in such widespread problems. (You’d expect that they’d catch it before release.)