Радостная новость - Microsoft будет отдавать исходники некоторых своих сборок .Net Framework 3.5. И можно будет отлаживаться прямо внутри них!

"Today I'm excited to announce that we'll be providing
this with the .NET 3.5 and VS 2008 release later this year.
We'll 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)."

http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx