Well, it seems that Microsoft will release the .NET libraries (not all, but it seems the most relevants ) source code with the next release of Visual Studio 2008 late this year.

This will be useful indeed while debugging in VS, but also it would be very interesting to look into the source code to view how they did it, expecially the Windows Form related part.. (..but what about Silverlight? :))

Read more here: http://weblogs.asp.net/scottgu/…framework-libraries.aspx