If you're interested on Windows Workflow and its future, I recommend to watch the Kenny Wolf 's section from PDC.
I can summarize it in a word: revolution!! 
The actual Workflow Framework and runtime seems to be totally revisited and the news are summarized as follows:
- Activity
- Authoring is simpler and takes much less code
- Fully declarative workflows and activities
- Alignment across Expressions, Rules, and Activities
- Seamless Composition Across Flow Styles
- Runtime
- 10-100X Performance Improvements
- Full control over persistence
- Flow-in Transactions
- Partial Trust Support
- Integrates with WCF, WPF, ASP.NET
- Tools
- Designer Performance and Usability
- Rehosting Improvements
- Unified Debugging Experience
The actual .NET 3.0/3.5 Workflow will continue to work on the .NET 3.0 WF runtime, not on the new 4.0 WF system. You can use your WF activities within a 4.0 Workflow but these activites will not be able to fully take advantage of the improvements made to WF 4.0 (WF 4.0 functionality is a superset of 3.0/3.5).
The main WF Activity change is the declarative expression of data on the workflow and at activity scope. This means no code-beside and the translation in a more simple language is: If you want to be prepared NOW for WF 4.0, instead of using CodeActivity, try to encapsulate your logic with custom activities and this will help your activities move forward (I hope to don't see an "Interop" activity for interoperation between 3.X WF and 4.0 WF but something more "elegant" and transparent).
An interesting news again that Kenny has shared with us:
We are helping Sharepoint, Dynamics, TFS, CRM migrate. The Dynamics team found about an 80% reduction in code for their activity implementation due to improvements around complexity being managed by the new runtime (no Activity ExecutionStatus, simplified performing of async work), the larger palette of built in activities, and the data flow and declarative features.
Great news and changements are on the air...