From the 'yeah they do open source' files:

Microsoft is best known as a vendor of proprietary code and applications (like Windows), but it's important to remember that since 2007, Microsoft has had its own OSI approved open source licenses. Late yesterday, Microsoft announced that it was open sourcing its ASP.NET MVC (model view controller) under the Microsoft permissive license. Microsoft describes ASP.NET MVC as, a "closer to the metal" web programming option for ASP.NET that enables full control over HTML markup and URL structure, and facilitates unit testing and a test driven development workflow.

Basically it makes ASP.NET more like dynamic language approaches found in  Ruby on Rails. This new open source code will actually also benefit open source developers outside of Microsoft, most notably those that use Mono - Novell's implementation of .NET for Linux. (Mono was just updated this week too.)

"I am psyched," Miguel de Icaza, vice president of development platforms at Novell blogged. "Not only because ASP.NET MVC is usable in Mono and the code is licensed under open source terms, but also because I strongly believe that the same innovation, rapid adoption and experimentation that has happened with the new wave of web stacks will come to ASP.NET MVC across all platforms."