• I've enabled back a display that I previously disabled, and the wallpaper is now grey
    Lion's Dock application has a bug, where it will not redraw the wallpaper after a display has been enabled again.

    The only way is to force killing the Dock, what is automatically done on SwitchResX 4.2.5 and above. However, this has the side effect of maximizing any minimized window from the Dock.
    Sorry, this is a bug that Apple itself has to correct.