That's pretty weird that plasma-widget-menubar is already in tree while appmenu-qt is not. Please add it! There is a nice ebuild in one of overlays: https://raw.github.com/megabaks/stuff/master/x11-misc/appmenu-qt/appmenu-qt-0.2.6.ebuild I've tested it on amd64 and everything works fine. appmenu-qt should probably be a dependency of plasma-widget-menubar since the latter is pretty useless without it.
That is indeed strange! x11-misc/appmenu-qt has been added to the tree, and will be added as a dependency of kde-misc/plasma-widget-menubar once it is keyworded for x86.
By the way there is the same problem with gtk applications. But the situation is even worse as appmenu-gtk needs patched gtk+ (just as appmenu-qt needs patched qt<4.8). “stuff”-overlay has all those ebuilds with patches (and a nice meta-package for appmenu-related stuff). Is it possible to add them to the main tree too?
~x86 added
Dependency has been added. For GTK applications, please file a new bug - the GNOME team handles all that. + 15 Jun 2012; Michael Palimaka <kensington@gentoo.org> + plasma-widget-menubar-0.1.17.ebuild, plasma-widget-menubar-0.1.18.ebuild: + Add missing RDEPEND on x11-misc/appmenu-qt, wrt bug #420909 by Kirill Elagin + <kirelagin@gmail.com>.
I've opened bug #421231.