I just debugged through the .NET Framework assembly source code like the way I used to do MFC source debugging earlier, except that the symbols and source are downloaded while debugging ...It’s much easier with VS 2008, especially the symbol download for debugging ... Looks cool ...

Configuring Visual Studio to Debug .NET Framework Source Code

http://blogs.msdn.com/sburke/archive/2008/01/16/configuring-visual-studio-to-debug-net-framework-source-code.aspx