Scott Guthrie just announced the availability of the ASP.NET Model-View-Controller source code. And believe me when I say this - the ASP.NET team is doing their darndest to make sure this works for you.
Our plans are to release regular drops of the source code going forward. We'll release source updates every time we do official preview drops. We will also release interim source refreshes in between the preview drops if you want to be able to track and build the source more frequently.
You really need to read everything Scott wrote on this. For me, it is an exciting time when I can download the code and make fixes to it myself to get around something that does not work. And if you truly are a geek you'll be going through the code base to see how the ASP.NET team implemented the MVC pattern. You also have a mode of feedback on CodePlex!