thanks for the comment brian, if you noticed I started a new thread to discuss this further.
Please contribute. Do we really need these two approaches to build browser based Ui's?
Also another thing, an HTML5 document is basically a FlowDocument from a XAML perspective, and in order to build an App interface you have to work around this arbitrary FlowDocument model, while in XAML you do it natively.
I think where you'll see XAML used is in apps that could also function as stand-alone apps. The idea of an offline HTML app is stupid, imo. Apps that will be done in AJAX / HTML5 will be primarily designed for web use and conform to the FlowDocument model.