Our smb.conf file for Redhat 5.2 used netgroups in several places to allow
access to various shares. When we installed Redhat 6.1, these shares were
no longer accessible to NT users. When the netgroups were replaced with an
explicit list of host and users, the problem went away.
The man pages for smb.conf still state that netgroups will work.
This should be fixed in the latest samba builds;
it's a build time issue.