Not sure you need help for this any more, but since it was one of the first hits I got on Google, I'm putting this up for reference.
The solution is opening a terminal and running
Code:
xmodmap -e "keycode 22 = BackSpace BackSpace Terminate_Server"
Also, I had this problem not due to XGL (as I've read is the most common culprit), but due to running a custom xmodmap file to replace "," with "." on my numpad. If you are running a custom xmodmap, it might be worth it to have a look at the file and see what "keycode 22" is bound to.
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