This was written more specifically for Ubuntu Maverick and previous, though Natty is just around the corner.
Previous Ubuntu versions, by defafult, had the key sequence CTRL-ALT-BKSP (Control-Alt-Backspace) enabled as a way to kill the X server in case a full-screen program was frozen beyond recovery. This key combination was turned off in Lucid, but can be re-enabled.
To re-enable, go to:
System > Preferences > Keyboard > Layout > Options [Natty/Unity: System Settings > Keyboard]
Drop down “Key sequence to kill the X server” and check the box next to Control + Alt + Backspace
That’s it! The key sequence is now re-enabled.
Questions, comments, and feedback is appreciated, as always.