well, after some more playing around, it looks like I've found a solution
Right click on one of the desktops, and go to show -> rows -> 3! It looks like Automatic is the default, and automatically chooses 2.. I've done this and it's nice to have a 3x3 layout again! The only thing I miss is being able to drag windows from one desktop to another using the tray applet, like I could in Gnome.. but ah well.. Guess this will have to do! (I realize this can be done if I launch the 'pager', but I don't want to have to do that just to move a window.. it's easier to right click the window in the taskbar)