Exciting news for all .NET fans! You don’t have to use the reflectors (disassemblers) to explore the internals of .NET libraries.

Microsoft is planning to release the source code of .NET framework. Microsoft will begin by offering the source code (with source file comments included) for the .NET Base Class Libraries (System, System.IO, System.Collections, System.Configuration, System.Threading, System.Net, System.Security, System.Runtime, System.Text, etc), ASP.NET (System.Web), Windows Forms (System.Windows.Forms), ADO.NET (System.Data), XML (System.Xml), and WPF (System.Windows).  We’ll then be adding more libraries in the months ahead (including WCF, Workflow, and LINQ).  The source code will be released under the Microsoft Reference License (MS-RL).

You’ll be able to download the .NET Framework source libraries via a standalone install (allowing you to use any text editor to browse it locally).  MS will also provide integrated debugging support of it within VS 2008. 
Please refer Scott Guthrie’s blog  for more details.