Not XNA related (and I doubt it will include the XNA libraries) but Microsoft are going to be releasing the source to the .Net Framework libraries and, because Microsoft are nothing if not integrated, they will be making it easy to access the source from within VS 2008. Being able to read the source is nice but we've all been doing that for ages anyway with the aid of Reflector. Reflector only allows us to read the source, with the actual source files we will be able to step into the Framework library methods and see exactly what is happening to our data inside the debugger. I'm really pleased to hear this.
See Scott Guthrie's post for more information.