StackOverflow Comment: 

Combining WCF with WF (Workflow Foundation) seems like the best option here. Workflow Foundation gives you lots of goodies, including long-term persistence over the lifetime of your long-running process.

In .NET 3.5, it's possible to do so, but clumsy and a lot of work.

Here are a few links for this topic:

• http://channel9.msdn.com/posts/mwink/Introduction-to-Workflow-Services-building-WCF-Services-with-WF/
• http://code.msdn.microsoft.com/WorkflowServices
• http://msdn.microsoft.com/en-us/magazine/cc164251.aspx

With .NET 4.0, these "WorkflowServices" will be a big part of the new WF/WCF 4.0 package. You should basically be able to expose an interface for any workflow as a WCF service. Sounds very promising, haven't had a chance to try it myself.

Some links for the new stuff:

• WCF / WF 4.0 and "Dublin"
• http://blogs.msdn.com/murrayg/archive/2009/06/23/windows-azure-s-net-workflow-service-to-support-net-4-0-workflows.aspx
• http://channel9.msdn.com/shows/10-4/10-4-Episode-24-Monitoring-Workflow-Services/
• http://channel9.msdn.com/shows/10-4/10-4-Episode-16-Windows-Workflow-4/

Another great article can be found here: http://www.west-wind.com/WebLog/posts/67557.aspx