If you encounter the following error when you try to mount a cifs filesystem:
CIFS VFS: cifs_mount failed w/return code = -22
try to:
#emerge mount-cifs
I did not find any indications on the net that a separate package should be installed in order to use cifs.
Later Update: same problem in ubuntu, you need to install smbfs
#apt-get install smbfs
Same problem in Ubuntu was solved by specifying IP addresses instead of dns names to the mount command.