Under Ubuntu Feisty Fawn 7.04, compiz is the default desktop effect window manager.
When first enabled by using desktop-effects, there used to 4 desktop by default. After disabling and reenabling it, only one virtual desktop was left, therefore the cube plugin of compiz was not working anymore.

In order to make the cube plugins of compiz to work, the desktop needs to have more than "1" face to rotate.

Somewhow, after disabling and reenabling compiz, the desktop number changed to 1...

In order to get this number back to something like 4, you need to start gconf-editor, to start it, type the hotkey Alt+F2 and type gconf-editor, then go to /apps/compiz/general/screen0/options/ and change the value of hsize from 1 to 4.

People using the command line can use the following command:

$ gconftool -s /apps/compiz/general/screen0/options/hsize --type int 4

This will set up the number of desktop in your session.

Hope this helps!