This may be outdated but I've just found it :)
ScottGu's Blog reports that the Microsoft's .NET Framework Library Source Code is available for viewing and debugging with .NET 3.5 and VS 2008.
The first .NET Base Class Libraries to be released (with source file comments included) are:
- ASP.NET (System.Web)
- Windows Forms (System.Windows.Forms)
- ADO.NET (System.Data)
- XML (System.Xml)
- WPF (System.Windows)
New libraries will be released in the months ahead (including WCF, Workflow, and LINQ). The source code will be released under the Microsoft Reference License (MS-RL).
The .NET Framework source libraries will be available for download via a standalone install. and can be viewed with any text editor. It will also provide integrated debugging support from within VS 2008, enabling developers to step through and review the source.
To learn more about the source release plans and how the debugger integration works, please check out this Podcast that Scott Hanselman and Shawn Burke recently recorded. There is also a video by Shawn Burke on Channel9 that shows how integrated debugging support works in VS 2008.