Link from the Arch Linux forums: http://bbs.archlinux.org/viewtopic.php?id=73064
Apparently, the latest xorg-server package 22.214.171.1241-1 on arch disables Ctrl-Alt-Bksp on user systems even if the "DontZap" flag is set to "False" in their xorg.conf.
Ironically enough, the solution is...
... another line in xorg.conf to re-enable ctrl-alt-bksp.
Just a heads up, since Arch packages are a bit ahead... Since the update comes from upstream, I'm not sure whether it'll come to Karmic sooner rather than later... although I'm fairly certain Jaunty users won't have to deal with thisCode:Option "XkbOptions" "terminate:ctrl_alt_bksp"