In the last release of Ubuntu (9.04) the Ctrl+Alt+Backspace key sequence normally used to kill the X server was disabled by default. Apparently many people like to kill their X server this way so a workaround "dontzap" package was used to enable it. With the latest Ubuntu release (9.10, aka Karmic) it's even simpler to enable it.

To enable Ctrl-Alt-Backspace in latest Karmic release simply go to:

   System –>
     Preferences –>
       Keyboard –>
         Layout (Tab) –>
           Layout Options –>
             Key sequence to kill the X server –>
               (Tick) Control + Alt + Backspace.