With the final release of Visual Studio 2008, Microsoft is going to make the source for much of the .NET framework available. It will be under the Microsoft Reference Licence.
So what is included in this? The set of libraries initially includes the Base Class Libraries (System namespace, IO, Text, Collections, CodeDom, Regular Expressions, etc), ASP.NET, WinForms, and WPF . Microsoft will add to this list as time goes on.
There are two ways people will access the source code:
1) They will download a package with all the source, and then they will be able to install and browse locally
2) VS 2008 integration will enable developers to debug from their own source code into the .NET Framework source code. We'll provide symbols for our source on an internet-accessible source-server; to enable this experience, the developer needs to set up the URI for the server. This second option is really cool, and enables a kick-ass developer experience.
So why care? This will aid in debugging and seeing how stuff works! No longer do you need to reflect out the source code, instead, you can step through it and see where stuff is failing.