There is an excellent article on Portal 6 and the new composite application infrastructure. The article is in a pdf that can be downloaded here with sample code here.

Composite applications aggregate a set of components into a single, coherent entity. The individual components and the interactions among these components can work together to implement complex business logic as defined by a business user. Components of a composite application can be virtually any code artifacts including Java™ classes, portlets, EJBs, or plain old Java objects (POJOs). Meta data describes the behavior and configuration for each involved component such as a reference to a document library or a workflow definition.

The model is used in the up and coming release of the Lotus Expeditor 6.1.