Today Microsoft is releasing some of the source code to the .NET base class libraries!
This is exciting. Check out more info on Scott's blog:http://weblogs.asp.net/scottgu/archive/2007/10/03/releasing-the-source-code-for-the-net-framework-libraries.aspx
Now, the biggest thing that came to mind when I heard this is, "I've got Reflector, so why should I care?"
Reflector is a great tool, for sure, but it's basically reverse engineering the IL. Sometimes things aren't clear or necessarily consistent. It's great for learning things and diving into the BCLs to satiate some of that curiosity, but that's about as far as it goes.
By releasing the libraries for some of the BCLs (and, hopefully, more will follow), it allows for a deeper introspection into the code, including comments, original variable names, etc. The coolest part, I think, is that you can get true F11 debugging support and jump into the code to see what's going on.
Now, one of the interesting comments I came across was asking if this compromises security at all. The answer is, unequivocally, no. Obfuscation is never a substitute for security, hence that old joke, "security through obscurity."