In a ZDNet article entitled Microsoft warming to open source? an MS exec tries to explain how shared source is just like open source, and they are doing it because their customers want it, and love them for it. There are a couple of caveats though:
The Windows program is a reference-only license. So in the Windows team today we let you view the code and debug against it, but you can't change the code.
and
Can people access all the Windows source code?
No.
Of course, the best quote has to be:
Microsoft does not subscribe to security through obscurity.
Course, it's only fair to go read it for yourself! :)