On Ubuntu it used to be that ctrl-alt-backspace would kill X (the backend of the various graphical user interfaces on Linux). In an aim to be user friendly this is now disabled by default. This can be a real pain if X locks up, you can’t kill it nor change to a console.

On Debian and Ubuntu you can install the dontzap command which will allow you to kill X:

sudo apt-get install dontzap

Then run dontzap:

sudo dontzap -d

Or you can the following section to your /etc/X11/xorg.conf file (which is what the dontxzap command does):

Section "ServerFlags"
        Option  "DontZap"       "False"
EndSection

See Alberto Milone’s blog for more info:
http://albertomilone.com/wordpress/?p=335