Virtual Member Manager (VMM) is not a new feature--it debuted in WAS 6.1--but a useful one that hasn't been talked about much. What VMM offers is "A single model for managing organizational entities." It's a composite user registry; you configure it with multiple user registries (LDAP or otherwise) and it makes them look like a single user registry for WAS and its applications.
Here's a diagram from our docs that illustrates the parts of the VMM:
My fellow employee in ISSW, Paul Ilechko, has published an article on the virtual member manager: "Expand your user registry options with a federated repository in WebSphere Application Server V6.1." The article explains that what VMM implements is a federated user repository, a virtual repository. It includes this helpful diagram to illustrate the concept:
You can also check out this example: "Sample virtual member manager custom adapter for WebSphere Application Server Version 6.1." There's also some introduction to the VMM in the "Security Enhancements Overview" in the IBM Education Assistant docs for WAS 6.1.
So if you've ever wished WAS could use more than one LDAP, check out the VMM.