Ubuntu 9.10 Karmic Koala has ctrl+alt+backspace turned off by default. ctrl+alt+backspace is the key combination used to restart the X server. The reasons for this disabling are unclear. One good thing though, is that it is only disabled and not removed.

To re-enable the ctrl+alt+backspace behavior you need to edit two files. This will enable the setting for all distros with a  Xorg Server version 1.6.0 and lower.

Edit the /etc/X11/xorg.conf file as root. Add the section:

Section "ServerFlags"
Option "DontZap" "false"
EndSection

For Karmic Koala, with Xorg 1.6.3, you need to edit a second file. Edit the file /etc/hal/fdi/policy/10-keymap.fdi as root and add the line:

<merge key="input.xkb.options" type="string">terminate:ctrl_alt_bksp</merge>

That will do the trick.

It you are uncomfortable with all these editing, you can simply do it the Lifehacker way.

Go to System > Preferences > Keyboard Settings. Look for the Layouts tab, expand the “Key sequence to kill the X server”, click the check box next to “Control + Alt + Backspace”. See the post at Lifehacker here.