Only samba-3.0.2a-r2 works in my old network. If add the line =samba-3.0.2a-r2 to the 'world' file (this version of samba was already installed) and do emerge -u world when samba will be updated to the newest version. (But if I do 'emerge -u world' twice samba will be downgraded to the version 3.0.2a-r2 again.) So, '=' in world file does not working properly. I use portage-2.0.50-r9.
pinning in the world file is dead use the files in /etc/portage/ to manage this kind of thing see the portage(5) manpage