As you might have noticed already if you use Ubuntu Karmic, 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.
As a result, now it’s very easy to use your Desktop environment of choice (e.g. GNOME, KDE) to enable or to disable the Ctrl+Alt+Backspace shortcut. It is also possible to do it without KDE or GNOME.
I recommend that you do it from your desktop environment of choice (as you may find it easier):
Instructions here
P.S. Instructions for XFCE are more than welcome.