We are releasing the source code for .NET Framework base class libraries under the Microsoft Reference License, and the source code will be downloadable and viewable by anyone who accepts the license agreement.
In addition, Microsoft will introduce a capability in Visual Studio 2008 to allow you to debug into the .NET Framework source code using Visual Studio. With this capability, when you are stepping through code, you will be able to step into the source code for the .NET base class libraries.
This release will include the Base Class Libraries (BCL), Windows Forms, ASP.NET, System.Data, and WPF. BCL includes many of the basic classes in the framework including collections, string and text handling, IO,serialization, remoting, and others. We plan to include additional libraries into the set as time goes on.
ScottGu has the details.