I've created a share to be used by both Windows and Linux clients.  For sharing purposes, many directories are setgid.  If I create a subdirectory with a Windows client, the setgid bit is appropriately set on the created directory; however, creating the subdirectory on a cifvfs mount on a Linux box, the setgid bit isn't set.  This is with the default 'nosetuids' option; using 'setuids' actually just makes things worse, since created files always have my primary gid.  If I create a  file and directory in a setgid directory, with both Windows and Linux clients, the results look like this:
[parsledl@joshu dptest]$ ls -Fla
total 16
drwxrwsr-x  4 parsledl ITDocs_Users 0 May 18 13:23 ./
drwxrwsr-x 88 apache   ITDocs_Users 0 May 18 13:20 ../
drwxrwxr-x  2 parsledl ITDocs_Users 0 May 18 13:20 lindir/
-rw-rw-r--  1 parsledl ITDocs_Users 0 May 18 13:20 lin.txt
drwxrwsr-x  2 parsledl ITDocs_Users 0 May 18 13:23 windir/
-rwxrw-r--  1 parsledl ITDocs_Users 0 May 18 13:23 win.txt*

Note that 'windir' has the 's' in group perms, and 'lindir' doesn't.  Note that I also created a Linux-specific share of the same directory, and I get the same results.  Here are the share definitions for Windows ([docs]) and Linux ([lindocs]):
[docs]
# Allow group write on files & folders, needed for simple sharing (setgid)
   create mask = 7775
   directory mask = 7775
   path = /docs
   browseable = yes
   writable = yes
# Special version of the export for Linux clients
[lindocs]
# Let the client umask determine perms
   create mask = 7777
   directory mask = 7777
   map archive = no
   case sensitive = yes
   delete readonly = yes
   ea support = yes
   path = /docs
   browseable = yes
   writable = yes