samba - May 2012 - Enabling winbind idmap_hash module

Hello,
I am using samba 3.5 (Red Hat Linux 6) which comes with idmap_hash plugin.

I have put the following in smb.conf:
    workgroup = WORKGROUP
    password server = *
    security = domain
    idmap backend = hash
    idmap uid = 500-33554431
    idmap gid = 500-33554431
    winbind nss info = hash
    winbind normalize names = yes
    idmap_hash:name_map = /etc/samba/name_map.cfg
    template shell = /bin/bash
    winbind use default domain = false
    winbind offline logon = no
    winbind enum users = true
    winbind enum groups = true
    log level = winbind:3

An abstract of /etc/samba/name_map.cfg is:
ntadmins=WORKGROUP\Domain Admins


I restarted winbind.

The problem is that the following command gives:
# getent group "WORKGROUP\Domain Admins"
WORKGROUP\domain_admins:*:16777224:

Instead of something like:
ntadmins:x:503:

The same problem with getent passwd.

I noticed that whatever value I put  for idmap backend (event a xrong 
value), it does not change anything nor produces any error message in log.

Any idea?
Thanks a lot