Ok, forget what you learned about workflow in 3.5.
Windows Workflow 4.0
4.0 changes everything you thought you knew about workflow.
WF 4.0 represents a bottom-up rewrite with entirely new thinking. The gains are enormous: custom activities take centre stage, and authoring them is much simpler; workflows are entirely declarative; and there are three workflow flow styles that you can combine seamlessly. It's possible that you could see a 10-fold improvement in the time required to create and debug workflows, in addition to 10- to 100-fold runtime performance improvements. You'd also have better control over persistence. Additionally, there's support for partial trust, a completely rewritten designer, and a better debugging experience. One of the most important improvements is in the re-hosting experience. This allows you to create activities and to allow power users -- and others -- to build or modify workflows with the constrained set of activities you've created. Yes, this is possible in WF 3.0/3.5, but the necessary code is challenging, not to mention exceedingly difficult to debug.
The improvements to WF 4.0 come at a cost. Microsoft developed two inter-op strategies to help you over this hurdle. WF 3.0/3.5 will remain part of the framework and will run side by side with WF 4.0. This lets you manage the transition at a time that fits your organization's broader goals. The other inter-op strategy is an inter-op activity that allows you to use WF 3.0/3.5 activities in a WF 4.0 workflow. Workflows are a specialization of activities, so you'll probably be able to run entire 3.0/3.5 workflows inside WF 4.0 workflows.
Screenshot VS2010 B2