Since Ubuntu 9.04, the Ctrl-Alt-Backspace key combination to force a restart of X is now disabled by default, to eliminate the problem of accidentally triggering the key combination. 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 10.04
* 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 10.04
* 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".