does openldap on OS X come with a schema you can use out of the box like Active Directory? the killer feature about AD is that if you had it since Windows 2000 you can upgrade it every version and every LDAP enabled application that MS ships will work with it as long as you are running a minimum version which only changes once every 7 years or so.
and everyone is shipping AD integrated apps that work since the schema is pretty stable.
from the apple site http://www.apple.com/server/macosx/technology/open-directory.html
Seamless directory server integration.
The Open Directory server integrates easily into your existing enterprise directory systems. With Augmented Records, you can use your companys primary directory server whether Active Directory or Open Directory for all user and group lookups and authentication. With no schema changes, you can store any custom information needed for client management, calendaring, or collaboration services in the local Open Directory server.