gconf-editor
then navigate to:
apps>nautilus>preferences
and untick "show_desktop". Done! The wallpapers you assigned in compiz manager will now be drawn on the appropriate cube face.
Original link: http://ubuntuforums.org/archive/index.php/t-105517.html
One word of warning - you will no longer have icons on your desktop. Does anybody know of a way to get icons also?