Microsoft has officially released the source code for the .NET Framework libraries.  The following libraries have been released (.NET Based Libraries, ASP.NET (System.WEB and System.Web.Extensions), Windows Forms (System.Windows.Forms), Windows Presentation Foundation (System.Windows), ADO.NET and XML (System.Data and System.Xml).  LINQ, WCF and Workflow have not been released yet.

 You can download the source from the following location (Visual Studio 2008 QFE).  For detailed instruction to set up with VS 2008, visit Shawn's blog Configuring Visual Studio to Debug .NET Framework Source Code. 

 Cheers,

Carlos