From Compiz

If you click on one of your desktops in the Workspace Switcher in your Gnome Panel and the cube doesn't rotate automatically then you can create a script to fix it. Keep in mind you will have to run this script each time the Workspace Switcher doesn't work. Create the script like this:

gksudo gedit /usr/bin/fixws

Then copy and paste the following lines into your text editor, gedit in this case. Once you have done it you can save the file and run it by typing "fixws" into any terminal.

#!/bin/sh

# Compiz workspace switcher fix

gconftool-2 -s /apps/compiz/general/screen0/options/number_of_desktops "2" --type=int &&
sleep 1 &&
gconftool-2 -s /apps/compiz/general/screen0/options/number_of_desktops "1" --type=int

The "sleep" time is necessary for Compiz to notice the proper setting!

You can also open up gconf-editor and set the following key to "1" (without the quotes):

/apps/metacity/general/num_workspaces

This should fix your problem, good luck!