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.

The "Ctrl-Alt-Backspace" key combination which was used to restart X was disabled by default from Ubuntu 9.04 onwards.

By default Ctrl + Alt + BackSpace key combination in Ubuntu Jaunty was disabled.This shortcut key is used  to restart X.This tutorial will explain how to enable this key combunation.

The ctrl-alt-backspace key combination was used to restart X-server and was recently disabled because of a upstream (X.org) design decision:

Ctrl+Alt+Backspace (i.e. the shortcut which was used to restart the X server) has to be enabled in a different way with respect to previous releases of Ubuntu.
This is due to the fact that “DontZap” is no longer an option in the X server and has become an option in XKB instead.

The Ctrl+Alt+Backspace combo, which allowed killing the X server is now disabled by default in many major Linux distributions. Here's how to enable it in KDE.

Current default for X servers as shipped in various distributions is to not enable the traditional Ctrl-Alt-Backspace key combination to kill the X server. If you would like to re-enable this feature, you may do so in your desktop's Keyboard Preferences application.

Latest release of Ubuntu 9.04 codenamed Jaunty Jackalope has few annoyances for experienced users such as Indicator applet, No default Compiz/desktop effects for some Intel based motherboards, Update manager nagging continuously for updates, cant restart X server by Ctrl+Alt+Backspace key combination etc. This article will enlist the fixes or workarounds for these annoyances.

One of the many changes in xserver 1.6 is switching the DontZap option's default so that the ctrl-alt-backspace option does not trigger X to terminate unless the user deliberately configures it to do so.