if there is foo-1.0.ebuild and foo-1.0-r1.ebuild in portage and ~foo-1.0 in package.mask, only foo-1.0-r1.ebuild is masked (emerge -p foo returns foo-1.0)
=arts-1.1.0_rc5* also doesn't work (just for the record)
just cc'd myself
works here, so I assume this was fixed a long time ago ;)