Recently Scott announced some really exciting news for .NET developers that his team has been working to enable the ability for .NET developers to browse and download source code of .NET Framework Libraries and also with debugging support.
This feature will be available with .NET 3.5 and Visual Studio 2008, which is expected later this year.
Microsoft will begin with offering the source code for .NET Framework Libraries including:
How it Works?
In Scott's blog you'll find all the details and screens on "How it works", but it's as easy and simple like you were debugging and browsing the source code locally - step through, set breakpoints, inspect variables, etc.

Here are few links to know more about this feature.
I'm really excited and just can't wait to get my hand on it. (:-P)