Visual Studio 2010 (VS2010) is being re-written using Windows Presentation Foundation (WPF). I was just listening to Scott Hanselman's 165th podcast in which he interviews Noah Richards about the editor in VS2010. What impresses me is that they're building VS2010 using VS2010. I can only assume that the first couple of builds of VS2010 were done with VS2008 and then I guess that once they had a working version they switched.
This must be the ultimate dogfooding exercise. Building a product using the product.