Xamarin.Forms is designed to work with MVVM and data bindings: http://xamarin.com/forms
To what extend is XAML support in Xamarin.Forms? Can I use it to create multiplatform controls/views?
Xamarin.Forms supports all of XAML 2009 and uses the XAML syntax with its own namespace and types. So, no existing XAML designer will work with Xamarin.Forms. However, the names and properties should be familiar to any experienced XAML developer.