there is also rAppid:Js http://www.rappidjs.com which is XAML based, but it's not made to work with Visual Studio or Expression blend.
A XAML framework for HTML/JS is a fantastic idea, XAML files could be converted in HTML5 or perhaps in JavaScript directly by having an InitializeComponents() method where all objects are instantiated (like in WinForms).
If you decide to start a such project, consider using a C# to JavaScript compiler like Saltarelle or Script#.