One of the OBA related news at TechEd Developer Barcelona was about the availablity of S+S BluePrints. These are open source frameworks designed to make it easier to build S+S applications. These frameworks include source code, guidance, structured step-by-step workflow and tools delivered within Visual Studio and can serve as a starting point for building real products.
The first of these BluePrints, Outlook S+S is now available. You can check it out at: http://www.codeplex.com/ssblueprints/Release/ProjectReleases.aspx?ReleaseId=8173