In Ubuntu 9.04 Jaunty Jackalope, the X11 restart keyboard shortcut alt-ctrl-backspace is by default disabled. This is done to reduce issues experienced by users who accidentally trigger the key combo.

However, I feel this shortcut is very useful and is a lifesaver in several situations.

If you want to enable this shortcut then either you have to edit the xorg.conf file or install a command named dontzap.

To install this package search in synaptic for package named dontzap or issue following command.

sudo apt-get install dontzap
Once you have installed this package, type following command in terminal to enable this X11 keyboard shortcut.

sudo dontzap --disable
For some reason, If you want to disable this shortcut again, then either modify the xorg.conf manually or issue following command,

sudo dontzap --enable
That's it folks.