Seems like Karsten Januszewski can't get away from writing designer tools. After shipping Glimmer: a jQuery Interactive Designer, he helped out with the Gestalt project and assisted with the XAML Playground.
In the XAML Playground, you can drag and drop XAML controls onto the design surface, see the generated code-behind and even copy it into your own Gestalt applications. Note, however, that the XAML controls provided are a very small subset of those available.