Xorg has an xorg.conf setting called "DontZap" which you can use to enable the Ctrl+Alt+Backspace key combo to kill the Xserver:
Nice double negation there. But it doesn't work in GNOME. Here is how you do it there: Go System -> Settings -> Keyboard -> Layouts -> Options -> Expand the triangle "Key sequence to kill the X-server" -> Check the checkbox. Took me a while to figure it out. In the future, it would be nice if GNOME could sync that setting with what is stated in xorg.conf.