If you haven't heard the news yet, you can read the full article on ScottGu's Blog

In summary Microsoft has released the source code for the .Net Framework in such a way that you can step into the Framework while debugging.  IMHO this is great for users because it allows us to get a lot of insight into parts of the API that are tricky or not well documented. 

Take a look at his blog for the full details of hooking this up into the debugger.