In my Post STS? Coming Soon! I linked to information about the upcoming framework for writing STSes (and more) from Microsoft. Unfortunately this is not yet available.
You can find the source code on Codeplex and a demo website here. Don’t expect a “click setup.exe and everything is fine” installation experience. There is still a lot of work to do. But you can get the complete source code along with installation instructions to play around. If you want to contribute or have feature ideas, leave me a comment.