I actually missed the release or the WCF activities for windows workflow a few weeks back but Marcel de Vries did a demo yesterday as part of his presentation at the SDE meeting. And I must say its an impressive piece of work that includes the whole enchilada, from sending and receiving messages all the way to generating the actual WCF class required in your service.
If you need to do any work with WCF and WF before the Visual Studio Orca’s release I can only recommend that you download this library and give it a good try. Of course with Visual Studio Orcas and the .NET 3.5 framework Microsoft will release support for WCF straight out of the box but for now this is the way to go.
And for that matter if you are interested in developing WF activities yourself taking a good look at these might prove a good lesson as there are a lot of things, like transactions, state management and persistence, to keep in mind when developing custom activities. Also if you are in the Netherlands and want to learn more about WF Activity development I suggest you come to the Microsoft DevDays in Amsterdam where I will be hosting a Chalk&Talk about the subject and Marcel will be presenting on WF as well.
You can download the WCF activities for windows workflow from: http://www.codeplex.com/WCFWorkflow