I had my head down yesterday recording a few screencasts and missed this announcement - the .NET Reference Source project has launched. That means you can now step into the Framework source code as part of your debugging session in Visual Studio 2008. Shawn Burke has all the details of how to enable this on his blog. There's also a great FAQ at the end of his post.
I'm not implying that the Framework needs to be debugged - but now, if you need to understand what