It’s been awhile since Ubuntu disable the possibility of using Ctrl-Alt-Backspace to kill the X Server. Here’s what you need to do to get that functionality back:

UBUNTU 9.10 (and newer)

OPTION 1
Edit the file /usr/share/hal/fdi/policy/10osvendor/10-x11-input.fdi, add the following in the section: <match key=”info.capabilities” contains=”input.keys”>:

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

OPTION 2
If the above doesn’t work, then add the following in /etc/default/keyboard:

XKBOPTIONS="terminate:ctrl_alt_bksp"

UBUNTU 9.04 (and older)

Add the following at the end of /etc/X11/xorg.conf:

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