Yesterday afternoon at the CIFS Workshop we had a meeting to discuss Samba 4's use of LDAP going forward, and what obstacles remained. Among the attendees that I can remember were Andrew Bartlett, Andrew Tridgell, Simo Sorce, Stefan Metzmacher, and (one more, I've forgotten the name) from the Samba team. Nicole Jacque and another (sorry, don't remember the name) from Apple/OpenDirectory, Pete Rowley from FedoraDS, and myself and Marty Heyman for OpenLDAP and Symas.
The upshot is that both the Samba and the LDAP sides have work to do, but there are no major roadblocks. LDAP will be Samba 4's default/recommended data store. As for OpenLDAP, most of what Samba 4 needs is either already implemented, or in progress.
Schema design tends to still be a stumbling block; in a separate conversation we discussed some design issues in MIT's new Kerberos schema as well as missing features in Heimdal's existing Kerberos schema. That's a bit outside this openldap-devel scope but I've committed to working with the Samba and Kerberos communities to draft some changes to unify these two Kerberos schemas.