Actually, the problem you mentioned is caused by the BIOS. I have the same problem. The BIOS interprets ctrl-alt-backspace as a command to switch off the computer. To kill X I have to switch to another terminal (if you're using GDM only 6 will be open, otherwise use ctrl-alt-Fsomething), ps -axu | grep X and then kill that process. it's the only way i've found. There are apparently settings in your BIOS that can stop the ctrl-alt-bkspace shutdown under APM options or something similar, but i could never get that to change anything, and i got sick of disk checks every subsequent boot. Anyway, its so rare you have to do it that it doesn't really matter.
Oops, i've just seen that you're wanting non-root users to be able to kill X. In that case, investigate the BIOS options for your system. sorry... hope i was some kind of help.