Comment 7 for bug 92780

Some additional info (as well as two temporary solutions):

If the gdm script (this is the script being executed when we press ctrl-alt-backspace) is launched in a terminal window, we have the same behaviour: no restart.
However, if the gdm script is launched in a console the restart works:

In Gnome, open a console (for instance ctrl-alt-f1)
Manually launch the restart script (sudo /etc/init.d/gdm restart or sudo /etc/init.d/gdm stop and then sudo /etc/init.d/gdm start).

Another possibility is of course to just logout and from the login screen press ctrl-alt-backspace.

I'll keep investigating why this behaviour (for the time being I have a suspicion only......).