Microsoft is changing their strategic direction with Active Directory. It's no longer supposed to be the application directory and configuration repository for everything, but will focus on authentication and providing a directory for computers and printers of a network. This is a reversal of the approach Microsoft took with Exchange 2000 - extending AD with loads of Exchange-specific schemas and storing Exchange configuration data in AD. Microsoft will provide a new directory called "AD/AM" - Active Directory Application Mode, which provides an LDAP interface and allows applications to store their configuration and directory data separately from the central company directory, but can at the same time integrate with the central company directory for authentication.
This strategy change is sort of ironic, because currently Exchange 2000 is forcing us to migrate the Exchange directory to AD, and now Microsoft discovered that this might not be such a good idea and they are working on an alternative approach ... However the Exchange 2003 (Titanium) Release is still built on AD. Btw, the Titanium release will work with Win 2000 and Win .NET. Exchange 2000 does not work with Win .NET directories.
Another interesting change is coming with Exchange Titanium Release. It will support a new replication protocol with the new Outlook 11 client that puts significantly less burden on the server and should therefore allow Exchange Server consolidation!!!
Another part of Microsofts new strategy is MMS - Microsoft Metadirectory Services. This is a SQL based database and allows replication between AD forests, AD/AM directories and foreign directories like Lotus Notes. It's possible that future versions of AD will also use SQL for storage.