Version:            (using KDE 4.2.2)
OS:                Linux
Installed from:    SuSE RPMs

With KDE 3.5 you could get the file name of the current background/wallpaper via the following dcop call:

  dcop kdesktop KBackgroundIface currentWallpaper 1

Could we add the equivalent to dbus?

Note: Although the file .kde4/share/config/plasma-appletsrc does have the background which you could grep out, it is not updated when you are using a slideshow, so only shows whatever you were using before you set up the slideshow.