Today we've announced (through ScottGu) that we are opening up the source code (complete with comments) for some of the .Net Base Class Libraries.
The purpose of this is to enable richer debugging support inside Visual Studio 2008 which will be released later this year.
When you hit a breakpoint in Visual Studio 2008 on a call to a method in a Base Class Library, you'll be able to drill into the implementation of that method to find where in the method the call is breaking.
This will take away much of the guess work associated with why a particular call is failing.
For more complete information, head over to ScottGu's blog and take a gander.