It took me a while to come to this blog post from Bart. He wrote a very simple and easy to understand article on how you can invoke a workflow engine from a IIS / Self Hosted WCF service.
This is very useful from the layered architecture perspective since most service-oriented systems will not have the presentation layer calling the workflow engine directly. More often than not (depending on the complexity of the system), you will want to inject a communication service layer in the middle to handle the Workflow call.
Great article there, go read it if you're into this kind of architecture.