Matevz adds his weight on Workflows and Services...
Fellow _MS RD_ and well-known distributed systems expert Matevz Gacnik has a great blog to add on top of mine called: Windows Workflow Foundation: Exposing Workflows as Services. Trust me, Matevz is a lot more than Request-Response or what his blog suggested i-smile.
In it, he explains some of the ways you can do so and the pitfalls to watch out for. Of course, you can get around the "workflow runtime can only get loaded once per appdomain" issues by having it static to the service implementation class OR if you want only one instance of each - _WCF_ also gives you a singleton-like instancing mode as well - InstanceContextMode:=InstanceContextMode.Single.
(I would be interested to find out the naming convention to call it Single in the latest CTP instead of Singleton)
My tip highlighted here is not really about exposing workflows as services. It is more about how you can hook a workflow into a already-hosted WCF service as part of its configured behavior, if need be. For example, you may want to have a non-intrusive workflow for you to raise an event that calls into your defined HandleExternalMethod (called EventSink before) and then you may just terminate that activity. The reason is because the workflow thread doesnt return unless you called a WaitHandle.Set, which you can call when a workflow is completed or terminated.
Having said all that - Remember that most of the current implementations of Web Services today work on a Request-Response model and many more are betting on that it will remain like that for a long time. Isnt this one of the reasons we have long arguments of POX/REST ? Workflows, on the other hand, are made to handle long running work and therefore, you need to design and handle both properly as its design principles and most-used implementations do conflict.
Now, if you forsake the Request-Response model and think about the wonderful partnership and the options abound once you hit <OperationContract(IsOneWay:=True)> on top of the MSMQ transport, _WF_ + _WCF_ does look very delicious and promising indeed.
