As some of you might have noticed already, there's news straight out of Redmond that some .NET source code is being made public with the final release of Visual Studio 2008. Basically, you sign up to a 'Microsoft Reference License', and get to view source code for the .NET libraries (you can't re-distribute, re-compile or copy the source code at this stage).
Included libraries are the Base Class Libraries (BCL), Windows Forms, ASP.NET, System.Data, and WPF. BCL includes many of the basic classes in the framework including collections, string and text handling, IO, serialization, remoting, and others. There are plans to include additional libraries into the set as time goes on.
Scott Guthrie broke this news - for more info, check his post at http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx