after doing an emerge gnome-base/gnome and firing up gnome. I didn't get any icons in the panel/menu's/nautilus/etc. After re-merging medi-libs/gdk-pixbuf everything worked fine. People on ircindicated that the problems stems from the fact that gdk-pixbuf doesn't have a dependency on libpng.
already fixed in gdk-pixbuf 0.16.0-r1