Coordination remains one of the most talked about topics with respect to portlets. And one of the popular means of enabling portlet coordination is via the event model.
The Eventing (also referred to as Inter-Portlet Communication) feature in the Sun Java System Portal Server as well as the decoupled Portlet Container component built out of the OpenPortal community now has tools support in the form of an IPC Story Board feature in the NetBeans Portal Pack.
Check out Satya's blog entry introducing this feature, and especially the screencast that illustrates usage via a sample Shopping Cart application.
Also note that the upcoming new versions of the Portal Standards, Portlet 2.0 (JSR286) and
WSRP 2.0, will provide support for portlet coordination through more than one mechanism including events. Support for those will also similarly make it's way into the Portal Pack.