.NET Framework Library Source Code is available for your Debugging Needs
Finally! I've been waiting for this for ages and now its available. Scott Guthrie has more details. First impressions are that the way it works is a little clunky, the first download of symbols takes quite a while (System.Windows.Forms is a 10MB download as an example) and you have no progress meter with Visual Studio being unresponsive until the download is complete.
Still, its worth it for when you're interested to see how the framework works internally with step-ins. All-in-all a good move by Microsoft. Remember guys, its a read-only license so no copying! :)