Check the 'virtual' setting of your XF86Config (or possibly XF86Config-4)...could be that you're using a "virtual desktop" or whatever it's called. Very annoying "feature" which I always stayed away from, so I can't be more specific.
Furthermore, 'man XF86Config' ought to be a good idea, along with going through the documentation of the ATI drivers. Unless, of course, you already did that.