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.
Article Source Linux Journal
November 3, 2009, 8:58 am