The manpage files emerge.1.gz, ebuild.1.gz, ebuild.5.gz and most likely others are in /usr/man/man0 although they belong to their respective manpage section directories. This means that man cannot find these pages, a "man emerge" just says "no man page entry for emerge" and exits.
they are all installed in the correct place on my machines ... in fact, none of my machines have a /usr/share/man/man0 directory ... could you try re-emerging portage and see what happens ? also, what version of portage ?
Portage version is what was included with a recent snapshot of the portage tree from about a week ago. I don't have exact version number since I'm currently not on my home machine. As soon as I get home I will re-emerge portage, perhaps that helps indeed. If the problem persists I might find the time to investigate using strace.
If you can show some reason for this, then reopen.