Switching Workspaces in Gnome 2
 

Sorry if I'm being really stupid, but my head is done in with this....
I'm only new to Linux (mandrake 8.2 originally) and had my workspaces setup so that moving the mouse to the side of the screen for half a second or so would switch to the next workspace.
I've since upgraded to Mandrake 9 and Gnome 2 and I can't find anyway to get the same thing to happen. I've got workspaces setup and I can switch with keyboard shortcuts or clicking on the applet in the panel but not by moving the mouse to the edge of the screen...surely I'm missing something.

Help anyone?