samba - Oct 2008 - setgid bit disappears

I have a samba server (without acls for better performance in ext3) with
shares where users create new directories and put their files.

I need the setgid bit to force the new directories and files to belong to a
shared group of users (not the primary group of the user that creates it).
When users create new directories the setgid bit is inherited and works
fine.

Users can set permissions for their new directories through the Windows
interface. But unfortunately then the setgid bit disappears.

I realise that the problem is a matter of linux (perhaps a security
issue).
>From the console i do:
 # su user
 $ mkdir directory
 $ exit
 # chgrp new_group directory
 # su user
 $ chmod 2770 directory
 $ ls -ld directory

 drwxrwx--- directory   (setgid bit is not there)

And if i do chmod 2770 as root  setgid bit is there.

It seems that isn?t a matter of Samba. I apologize for the inconveniences.