I think this would be much more interesting if you could provide it as a product (i.e. LDAP in a box) rather than purely as a service.
The Platform as a Service angle is very interesting, and we're planning to release an addon for the major platforms such as Heroku, GAE, OpenShift, and CloudStack. CloudStack is already setting a precedent with their new LDAP authentication functionality. We'll see if it catches on.