Metablogging via ScottGu and InfoWorld: Very good news - with Visual Studio 2008 and the .NET Framework 3.5, the source code of the .Net Framework libraries will be made available via the Microsoft Reference License which will make debugging possible right into the Framework classes.
Quoting ScottGu, first, 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) will be made available, then, be more libraries will be added in the months ahead (including WCF, Workflow, and LINQ).
The .NET Framework source libraries will be obtainable via a standalone install and there will be integrated debugging support within Visual Studio 2008.
I urge you to take a look at Scott's post on the subject as it contains some good in-depth information.
Read: .NET Framework Libraries Source Code releasing under Microsoft Reference License