Microsoft announced they will release ... the source of .NET Framework 3.5 and we will be able to trace how our data is handled by .NET base libraries.
Also it would be very good for education purposes for young devs.
And, in the manner they say in TV Ads, this is not all. You also get access to Program database - .pdb file - where debug symbols live so the VS 2008 IDE can match the code that is executed with the line is source code :)
So who says Microsoft is not open company?
(via ScottGu blog)