[wlug] GNOME Workspace issue

I've suddenly noticed that I'm unable to switch between workspaces. Neither clicking on the workspace applet nor Ctrl+Alt+Left/Right arrow work any more, and I'm at a loss as to why this might be. I can still send windows to another workspace, but I can no longer get to them. Would anyone have any ideas about why this is happening? This is 11.04 x64.