Active Directory secondary to Open Directory?

I'm competing for the directory services assignment described below but have not yet spoken to the client. As you can see the client is considering making Open Directory the primary directory service and integrating Active Directory secondarily to support Exchange clients. I've never seen a setup like this, or anything other than either Open Directory exclusively or OS X clients using Active Directory as the primary directory. Can someone give me some insight on how Active Directory could be made to work in the manner the client is considering? I'm sure this isn't what Microsoft had in mind :-) ------------------------------------------------------- Client is considering Apple 'Open LDAP' or Sun LDAP. Will consider any Linux based open source LDAP, although Apple LDAP is preferred for its impressive user interface. a. Install apple LDAP as primary accounts database b. Configure Active Directory as the secondary call for accounts database (still using Microsoft Exchange) ----------------------------------------------------------