Having source code access and debugger integration of the .NET Framework libraries is going to be really valuable for .NET developers.  Being able to step through and review the source should provide much better insight into how the .NET Framework libraries are implemented, and in turn enable developers to build better applications and make even better use of them.

To learn more about our source release plans and how the above debugger integration works, please check out this Podcast that Scott Hanselman and Shawn Burke recently recorded.  Shawn (who drove the source project on my team) is also going to be publishing a cool Channel9 video later this week that shows using the VS 2008 integrated debugging support with it.

Hope this helps,

Scott