The mambo CMS is already in portage (bug 55316 requested an ebuild, and bug 65499 is requesting an upgrade). However the current version, 4.5, is only marked unstable on x86. I have sucessfully installed it on my PPC, so I know there are no problems. Please add, at least, ~ppc (I haven't tested it enough to comment on whether it should drop the '~' flag...). Since it is just a simple PHP web app, it could probably be marked unstable on all platforms.
Done in CVS for ~ppc. Please bug the web-apps-herd for bug 65499. :)