I am trying to diable Disabling <ctrl><alt><backspace> in X. I don't want anyone to be able to shutdown my session when I lock the screen with a screensaver.

I have uncommented the line "DontZap" in XF86Config but this didn't fix it.

Any help would be appreciated.

This is pretty funny. I have the opposite problem of you. I cannot kill X with ctrl+alt+backspace . This is due a little problem I'm having with my keyboard layout in Slackware 9.0 and graphical mode. To correct the problem, I have to load the correct xmodmap. Loading xmodmap xmodmap.se (for my swedish layout) it corrects the keyboard and the backspace/killX thing stops working.
Try looking if this works for you. This is far the most inaccurate solution, but would be a "working around" untill somebody explanes why "DontZap" does not work for you and why my xmodmap kills my shortcut in killing X.

Go into /usr/share/xmodmap

find your keyboard layout there and type

xmodmap xmodmap.us (this is just a guess)

If it works, and you are satisfied with this n00b work around I did, copy your modmap to home directory using:

cp xmodmap.us /home/user/.Xmodmap

Good luck and sorry for not having a wiser solution

Just so everone knows:

I added a ServerFlags section to my XF86Config-4 file containing the DontZap line.