With a lot of help from Google and a good howto for another Linux distro, I last night got OpenLDAP up and running on one CentOS 5.2 virtual server, and 3 other servers working as LDAP clients, one of which is now running as a Samba PDC with LDAP back-end. Added some users, set passwords, and Samba is now authenticating nicely against LDAP for file access from my Windows PC. Not using the PDC functionality yet (my kids need Fast User Switching), but so far so good.
I am using Webmin and PHPLDAPadmin to manage it all. I will post more detail over the next week or so.