At last year’s LDAP-Con event, Ludo from OpenDS and Howard from OpenLDAP presented on the work that they’d done on using MySQL Cluster as the scalable, real-time data store for LDAP directories (going directly to the NDB API rather than using SQL). Symas now provide their implementation (back-ndb) for OpenLDAP.
You can view the charts at http://www.mysql.com/customers/view/?id=1041