i had this happening to me also on Snow Leopard, i have a nvidia GTX 260 card, and found that when i turned my monitor off then on again, (straight away) i got my desktop back also the other trick i used was use the keyboard shortcut to log out (command+shift+q) waited a 1min for it log out and then i got a normal desktop. But then i found a fix for me, changed DVI ports on my video card, so i two DVI out and changed ports rebooted and away i went! no more problems. Hope this helps, it seems like a common problem lately.

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im a PC running Snow Leopard