OK, ATTENTION! There is a new app I made which is much better than this. see here: http://ubuntuforums.org/showthread.php?p=5581392
So please ignore this hacky script and use the app.
I've written this small python program which picks out a random background from your list of gnome backgrounds; including all it's settings (colours, options, transparency etc) and sets it as the current settings.
I've done this for a friend of mine who wanted a KDE like wallpaper option, I've created this as an icon and as a per hour using the Schedual program.
all good fun. send patches if you like.