We announced last month that we'll provide the ability for developers to download and browse the source code of the .NET Framework libraries, as well as enable integrated source debugging of them using Visual Studio 2008.  You can learn more about this in my blog post here.

We are finishing up the final deployment of the source servers that host this now, and will be publishing instructions on how to enable the integrated debugging experience within Visual Studio 2008 shortly.  I'll blog detailed steps on how to turn this feature on once it is available.