On amd64, installing app-text/acroread should tell the user (or perhaps do it for them) they can run `nspluginwrapper -i /usr/lib32/nsbrowser/plugins/nppdf.so` to get Acrobat Reader's 32-bit nsplugin working on 64-bit (though only if nspluginwrapper is installed obviously). You'll want to `nspluginwrapper -r /usr/lib64/nsbrowser/plugins/npwrapper.nppdf.so` on uninstall though. A message post-install would be sufficient; actually running (USE-based?) would be awesome.
Thanks for your suggestion, in CVS in >=acroread-8.1.4.