There is a rehosting sample called "Workflow Studio" available here: Workflow Studio for WF 4.5. There is also a blog entry: Workflow Foundation (WF4) – Rehosting The Workflow Designer

a simple, generic application that allows you to design and execute multiple XAML based workflows in a windowed environment akin with Visual Studio. Use it to design and test your workflows in environments where Visual Studio is not the right tool. You can even use it as a simple hosting environment. I’ve developed Workflow Studio as an application you can use out of the box, or you can use it as the basis for your own specialised implementation.