Last night I followed Scott Guthrie's instructions as to how to get early access to the .NET Framework Source Symbol Integration with VS.NET 2008. Kudos to Scott, Shawn Burke and team for going through the effort to make this feature available to us all.
This is such a great feature to have for two reasons. First, is that it's going to make debugging so much easier. Stepping into the framework source, seeing exactly what is going on as you step through is huge. Second, there's a ton of great programmers working on the framework. Being able to read their code exactly how it was written (Reflector reverse engineers it out of MSIL after compiler optimization) is a great way to become a better developer.