Illegal aliens have always been a problem in the United States. Ask any Indian. - Robert Orben
This was trumpeted several months ago mostly by Scott Guthrie, and now it is live (although not all sources and symbol files are available at present).
None of this is rocket science, it's already built into Visual Studio. However, up until now there were no symbol files and sources for the BCL and related assemblies. Now there are.
This is, in my opinion, not only a great debugging tool for any developer who is interested in "getting under the hood", but it is also a great learning tool. If you are not sure you understand what I'm talking about, just go ahead and "do it".
The easiest way to get started with this is to follow the instructions on Shawn Burke's blog post here.
Be sure to follow the instructions exactly - they are not difficult. It requires the download of a QFE to set up the debugger properly, the url is specified in Burke's post.
There is also a new forum hosted by Rob Conery here. There weren't any posts yet as of the time of this writing, but I am sure there will be!
And for those who understand the deep link between programming and jazz, I offer you Bill Evans, who once said:
"To the person who uses music as a medium for the expression of ideas, feelings, images, or what have you; anything which facilitates this expression is properly his instrument."