Enable Ctrl-Alt-Backspace to Restart x-server (from Ubuntu wiki)
One of the nicer things about Linux is that you rarely need to reboot. Sometimes however you might need to restart the x-server. In older Ubuntu releases this could be dome with the Ctrl-Alt-Backspace key combination. Since Ubuntu 9.04, this key combination has been disabled by default, to eliminate the problem of accidentally triggering the restart. In addition, the Ctrl-Alt-Backspace option is now configured as an X keymap (XKB) option, replacing the X server “DontZap” option and allowing per-user configuration of this setting.
As a result, enabling or disabling the Ctrl+Alt+Backspace shortcut can now be done easily from the desktop.
Enabling Ctrl-Alt-Backspace for Ubuntu
Select "System"->"Preferences"->"Keyboard" Select the "Layouts" tab and click on the "Layout Options" button. Select "Key sequence to kill the X server" and enable "Control + Alt + Backspace".
Enabling Ctrl-Alt-Backspace for Kubuntu
Click on the Application launcher and select "System Settings" Click on "Regional & Language". Select "Keyboard Layout". Click on "Enable keyboard layouts" (in the Layout tab). Select the "Advanced" tab. Then select "Key sequence to kill the X server" and enable "Control + Alt + Backspace".