As stated on ScottGu’s blog in his latest post, the .NET Framework Library Source Code is now available. A detailed blog post about the steps to follow to enable .NET Framework source access in Visual Studio 2008 can be found at Shawn Burke’s blog in this post.

Taken from Scott’s post:
Once you follow the configuration steps in Shawn’s post above, 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.