Scott Guthrie announced yesterday that Microsoft will be releasing the source code for the .NET Base CLass Libraries, Windows Forms, ADO.NET, XML, and WPF in the near future to enable easier debugging support. This is excellent news and will surely be a timesaver for many developers.
Now, before you ask: The XNA team has no current plans to release source code for the XNA Framework or other proprietary tools and runtimes. That doesn't mean it can't happen in the future (never say "Never" ), but it's not on the table right now.