Recently, I have this annoying problem in Ubuntu 12.10 with Gnome-fallback: My only workspace width has become twice the size of my monitor (a 19" Philips). Because of this, some applications, like gnome user interface, go to to the right part of the workstation and become reachable. I have installed CompizeConfig Settings manager and set all Desktop sizes to '1'. but it did not sove the problem.
I really appreciate your hints to fix this.