As I've written earlier, Visual Studio.NET 2008 makes it possible to debug code that can be found on a source server.
I think this can be very interesting and I can think of numerous situations where this can be very handy.
Suppose you work in a company that uses an in-house developped framework, and you're building an application that uses this framework.
If you experience some strange behaviour inside your application, you can debug your code and step into the code of the framework to see if the company's framework has a bug.
I've read that Microsoft has setup a source server which contains the debug-symbols of the .NET framework, so, as from this month, it is possible to step into the .NET framework source-code as well!
Setting up VS.NET 2008 to enable this is very simple; you can find a step-by-step guide here.
The Mozilla team also have a symbol server. You can read more about it here