Ubuntu 9.10 (and newer)

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">:

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


Ubuntu 9.04 (and older)

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

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