I noticed this immediately upon upgrading to Ubuntu 13.04 on my main home computer.
Wallch appears to be running correctly, but you only see the default wallpaper.
Looks like, for whatever reason, I had to fire up the gnome tweak tool, and set "Have file manager handle the desktop" to ON
Looks like whatever method was working previously won't do it anymore, and that nautilus needs to be handling wallpaper now in order for this to work.
The only "down side" of this that you now see desktop icons that previously were not visible.
Honestly, this is a setting that I have always changed (to off) on any ubuntu/gnome-shell install I do, but for some reason that I have completely forgotten. So, long story short, I guess I won't be doing that any more.