The .NET Workflow Service has been part of the .NET Services platform for multiple CTP releases. Its architected to be the "cloud" Workflow hosting solution. The .NET Workflow Service was built on top of WF 3.5. Lately, we made a decision to build on top of WF 4.0 to take advantage of the enhancements WF 4.0 provides and because having a Workflow solution built on top of the latest framework version is what customers have been asking for. As a result, we will hold off releasing .NET Workflow Service as part of the .NET Services platform until after .NET Framework 4.0 ships. Please read the post here for more information.