I've been spending some time working with hosting the WF runtime in a WCF-wrapped Windows service. The idea is to allow these services to be run on servers behind a load balancing technology like NLBS. In my research I ran across a few great articles and samples which might be useful so I thought I'd post them here.
My goal for these hosting services is to make them as loosely-coupled as possible. Ultimately, I'd like to have the full set of runtime and service events available through a publish/subscribe architecture similar to Juval Lowy's so that the disconnected and potentially remote consumers could be notified of status and process changes in their respective workflows. Also, to accomplish the loose coupling, I am taking bits and pieces from the Workflow Adapter/Connector and WS-Transfer Service for Workflow articles that Roman Kiss posted on CodeProject.com. There's a lot of work to do but I think in the end the architecture is a sound one. I'm not sure I'll go all the way to WS-Transfer yet but it's definitely a compelling protocol.
There's a good example of hosting workflows in a Windows service out on the Workflow community site. It also demonstrates the use of IExtension in WCF for hosting the Workflow runtime.