Using 12.04 with unity 2D I have the opposite problem to this question - how can I change from a 2x2 arrangement to having a single row of workspaces?
Turns out the work-around listed to use gnome-panel does still work in 12.04. It appears to be only necessary to install gnome-panel:
The other packages listed are not needed. Once installed, its justs a matter of running gnome-panel on the cli, using its preferences to set the required layout and existing gnome-panel.