Debugging into .net source code
You may be aware of the availability of .net source code for debugging. Scott Guthrie had mentioned this along with release of VS 2008. Though Reflector still remains my personal favorite for quickly checking the .net source code during my regular work, it can't help much during debugging.
Recently the .net 3.5 SP1 code was also made available. In case you haven't checked this out, here are the instructions on how you can configure and use this feature from VS 2008. I tried this and it is awesome to be able to debug straight into .net source code and see what is really happening in there. Do note, however, as Shawn also mentions, the initial load time can be pretty high and i had to wait almost 30 min before I could start on with debugging. However since the symbols and the .cs files are cached locally, it is very fast thereafter. Go ahead and give it a try !