Centralized and Secure Autentication?

signal7 asks: "I run a network of 50+ workstations. Some are UNIX, some are Novell, some are NT, etc. The problem is I recently setup a killer Samba server, but synchronizing user information in a *SECURE* fashion just doesn't seem possible. I'm looking for something that provides the exchange mechanism of say Lerberos, with the centralization of LDAP. Anyone have any suggestions?"