Comment 1 for bug 119070

I've seen this happen before.. I came up with a simple fix for it.. not sure if there is a better one..
but goto System > Preferences > Desktop Effects

Now while that is up right click on your workspace selector in the bottom right corner and click preferences

The number of workspaces will say: workspaces: 1
change it to four or whatever then uncheck and re check the checkbox that says "Workspaces on a cube"

You sgould get your cube back