The .Net Framework 3.5 Source Code will be released later this year.
(announced in http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx)
This isn't going to be a major revolution since the code was already "visible" using tools like Reflector.
But now we will have the complete detail of the source code (private variables names, comments ...) and we will also have the possibility to debug it with Visual Studio 2008. So we can understand better what's "under the hood" and in a much easier way.