After installing Gnome 2.18, a lot of icons didn't show up in my menus. After searching on google I found out, it had to do with the gtk icon cache. Apparently, the function gnome2_icon_cache_update() in gnome2-utils.eclass doesn't update correctly. If I force the cache update, everything works, but the function doesn't force it. To force updating, the updater needs to be called with the -f option.
I can confirm this bug, in my case the pidgin icon didn't show up in the gnome menu, but after running gtk-update-icon-cache -f /usr/share/icons/hicolor, it appeared just fine.
I've reverted the changes. This is a work in progress. *** This bug has been marked as a duplicate of bug 155993 ***