Version: (using KDE 4.0.83) Installed from: Ubuntu Packages Currently Compiz can paint natively on every desktop a different wallpaper editing the value of the containment wallpapercolor=0,0,0,0 in ~/.kde4/share/config/plasma-appletsrc ... If in Desktop settings is selected the option "none" to the wallpaper the desktop is black, ok. I suggest include a "transparent" option down the "none" that disables every option as "none" does AND the color selection button (when you choose "none" you can pick a colour for the desktop), and save in the containment the wallpapercolor value to 0,0,0,0 (in ~/.kde4/share/config/plasma-appletsrc should I have finally wallpapercolor=0,0,0,0 ) Then when Compiz is used instead of kwin will show every wallpaper that Compiz Fusion has configured for every viewport. If kwin is still used with "transparent" wallpaper option will work exactly as the "none" wallpaper option, painting the desktop as normal (by default I think that is a black color).
having the window manager paint the wallpaper is .. to be frank .. braindamage. i understand why compiz is doing this: no desktop shell out there is smart enough to keep up with ideas like "showing more than one virtual desktop / workspace / viewport at a time" we have a solution for this in plasma that is partly complete and will be finished in 4.2. however, due to other features in plasma which are in conflict with the idea of an externally painting wallpaper, plasma will not be offering a "no background" option at this time or in the foreseeable future. (btw, i have been talking with compiz developers about this issue.)
Thanks by the reply, is good to hear that something better than a simply "workaround" (as I said) is being planned to future KDE 4 releases :-)