It's pretty interesting to see how useful workflow services can be sometimes, specially for business applications that require orchestrating several back-end services calls.
As one of the programmers involved in the development of the WS-I Sample application for .NET, an application that demonstrates how to build interoperable web services in .NET using the WS-I Basic Profile 1.0 (Each vendor in the WS-I working group basically implemented the same application using its development platform, more information can be found here), it was a surprise to me to find another implementation as part of the WCF SDK, made this time all declaratively through WCF workflow services and just a few activities. To give you an idea, the original version of the same application took us some days and hundred lines of code just to have something complete and working.
The SDK version is available under the folder, [SDK Folder]\WCF_WF_CardSpace_Samples\WCF\Scenario\WorkflowServices\Conversations
Workflow services, initially introduced in WCF 3.5 with the Receive and Send activities, will now take a very important role as part of the Oslo vision. We will have the opportunity to write pure declarative long-running services with XAML, including all the WCF contracts for receiving/sending messages. This will make possible to store the complete workflow representation in the Oslo repository, and take later full control of this workflow's instances from Dublin, the new hosting environment for long-running services. David Chappell has written an excellent article about WF 4.0, Dublin the oslo vision, you can find it here, this a must read for developers interested in this area.