I had the same issue in Xubuntu. I found my answer here: https://sites.google.com/site/easyli.../first-xubuntu (<--- solution credit goes to this author. His instructions pasted below for ease of use)
I think that this should work in Ubuntu just the same.
In a terminal enter:
gksudo leafpad /etc/default/keyboard
Find the line:
Change that line to this:
Save the modified file and close it.
Reboot your computer.
After logging in again, you can test it: Ctrl+Alt+Backspace should reboot only the desktop and throw you back into the login window.
For me if XBMC hangs - now I can kill it by hitting Ctrl+Alt and Backspace. Log back in and it brings me back to my desktop.