Mac OS X has a kind of different client behavior with a SMB-Server (Samba 3.6.6-6) than a Windows client.

By saving new files, they will be stored with Mask “0600/-rw——-” and that doesn’t matter what samba configuration prescribes (force create mode = 777).

So the explanation for this problem was found in the SAMBA documentation:

SAMBA has the “unix extensions” option set to “on” by default. This allows Unix users who write to the Samba shares to set their own permissions bits.

Now set them in the SAMBA global section [1] to off:

1

unix extensions = no

[1] SAMBA configuration file: /etc/samba/smb.conf