It is something I'am missing all the time while working & playing - a hotkey for toggling the fullscreen option.
This thread at the Xonotic Australia forums may describe close to what you're after, if you run Linux.
Thanks a lot - this is exactly what i was looking for!
The only problem I see is that the option mentioned there: vid_netwmfullscreen doesn't exist.
It seems to exist only in the GLX client.