Getting a consistent experience with design-time data in WPF (Part I)

Recently, I have been trying to populate my WPF design-time controls with mock data in order to have real-time feedback of the UI, as I am tweaking the XAML. I first saw this in action at a session with Richard Griffin, Felix Corke, and Ian Griffiths at Tech Ed Barcelona 2008, and I was blown away by the prospects of it. This has been the Holy Grail of form design for me, ever since the first releases of Visual Studio came out (think Win32, MFC, Windows Forms), and now it seemed Microsoft nailed it!