There is a very interesting article out from Microsoft all about hosting the Workflow runtime behind a Windows Communication Foundation service layer. There are some great points on using duplex channels so events from the runtime can be sent back to the client. The design is fundamentally similar to what I've been working on for a few clients as described in this post.
It appears that this is an evolution of the Expense Reporting Sample that's been out awhile. The first version used .NET Remoting instead of WCF. Guess they didn't want to use someone else's alpha code to build their beta code 
Check it out.