I hear the term XAML user interfaces mentioned way too often these days.
XAML has in fact nothing to do with userinterfaces. XAML is just a language for instantiating an object structure. I have previously written about how I have used it as a configuration language for my applications, a sort of light weight dependency injection framework. To show that WPF and Silverligth are not the only UI frameworks that can be used with XAML I have used XAML to create a small Windows Forms form.
The root node of my XAML file will be Form. By providing "clr-namespace:System.Windows.Forms;assembly=System.Windows.Forms" as the default namespace Visual Studio and .net will recognize the elements and validate them to match a structure that is allowed beneath a form. If you need to use other .net namespaces, you just add a prefixed xml namespace for each of those .net namespaces.
Visual Studio will pr default open a WPF/Silverlight visual presentation along with the text editor when opening the XAML file. As we are not using WPF or silverlight the visual presentation will not show anything usefull. The XAML can be distributed as a file besides you application, embedded as XAML or embedded as BAML (XAML compiled to a binary format). The BAML approach is probably the most sensible solution for a real life application.
To create an instance of the form just give the XAML stream to a XamlReader and cast the result to a Form object. Now the form can be displayed by calling Show, like you would on any other windows form.
When the form object has been created, i use two lines of code to hookup some eventhandlers before showing the form. The find method on the form is an extension method for Form that flattens that flattens the structure of the form so that LINQ can be used on it. The generic type is a filter that only returns the controls of that type and the lambda is a further filter. The find method has been implemented in the plain framework, so you can go there to se the implementation.
The Do method is just like the ForEach method on List, this however work for any type of IEnumerable<T>.
The first of the two lines finds all buttons whose name start with "Command" and hook them up to an event handler. The second line hooks an event handler to the TextChanged event of all controls whose name start with Value. In this way it is easy to use a naming convention to hook the UI up to business logic. Dependind on your architecture, you might hook it up to fire commands or whatever.
You might implement some layout manager the same way, or you might inherit panel or something other control and create a layout manager that way. By implementing you own controls you can also put metadata on the controls in your UI.
So, XAML has the potential of being used with other UI frameworkds than WPF and Silverlight.