Mark Schmidt has updated his fun WFPad application for WF Beta 2. It allows you to edit XAML in text and in the workflow designer at the same time to see how each affects the other. A great way to learn about declarative authoring of workflows. Mark is a programmer writer on the workflow team at Microsoft.
Moustafa Ahmed has written an article describing what states a workflow instance goes through in it's lifecycle. This is really useful in understanding what's happening at runtime to the workflows that you create. Moustafa is a program manager on the workflow team at Microsoft.
Israel Hilerio has written a technical introduction to Windows Workflow Foundation for Sys-Con. If you're new to workflow this might be the place to start. Israel is a program manager on the workflow team at Microsoft.