Last October, ScottGu blogged about releasing the source code to the .NET Framework libraries, and enable debugging support within Visual Studio 2008.  This is now available for everyone to use.

Check out the full post on ScottGu's Blog at: http://weblogs.asp.net/scottgu/archive/2008/01/16/net-framework-library-source-code-now-available.aspx (for which libraries are available, reference license, etc.)

Enabling .NET Framework source access within Visual Studio 2008 only takes a few minutes to setup.
Check out Shawn Burke's detailed blog post that covers the exact steps on how to enable this in more depth here. If you run into problems or have questions setting it up, please post a question in the Reference Source Forum on MSDN here.

Once you follow the configuration steps, you’ll be able to dynamically load the debug symbols for .NET Framework libraries and step into the source code. VS 2008 will download both the symbols and source files on demand from the MSDN reference servers as you debug throughout the framework code.

"Being able to step through and review the source should provide much better insight into how the .NET Framework libraries are implemented, and in turn enable you to build better applications and make even better use of them."