This is from a presentation I have done internally a few times and most recently at TechEd Barcelona:

The .NET Framework 3.5 will introduce the functionality to call services from Windows Workflow Foundation (WF), and to expose workflows as a Windows Communication Foundation (WCF) service. A common pattern is to have a workflow serve as the coordinator between a number of other processes (including workflows). This talk will discuss how these conversations are implemented in WF, and common patterns for conversing over a long period of time, including asynchronous messaging, long-running cancellable work, managing m-of-n responses and check pointing of progress.