After toying with Microsoft Virtual Labs (http://msdn.microsoft.com/fr-fr/aa570323(en-us).aspx) on Workflow Foundation (WF) in .NET Framework 4.0, I must say I’m pretty impressed.
Coupled with Windows Server AppFabric (formally known as Dublin Server), it’s basically BizTalk Server without all the schemas and adapters coming out-of-the-box with BizTalk. Otherwise, it’s as powerful and actually much easier to use.
The integration of WCF & WF, so half-ass in .NET 3.5 SP1, is quite well done. You create a Workflow, drop a few Send / Receive activities around and that creates an implicit WCF endpoint with an implicit WCF contract. Quite gorgeous!