You say most icons are missing? What type of icons are there, and which are missing?
I have a similar issue on Fedora 15. Xfce apps and "third party" apps have their icons in the menu list, including the Accessories menu, LibreOffice, Firefox, etc. However, icons from KDE and Gnome apps are missing, including gThumb, Konqueror, Ktorrent, and more.
For some of them, I've solved it by manually setting the icons on my panels. E.g. I found the gThumb icon under /usr/share/icons/hicolor/48x48/apps/gthumb.png