Use Reflector to determine what .NET namespaces are aliased to Silverlight XAML namespace

When you use a XAML element like <Button> in your Silverlight application you get an instance of the System.Windows.Controls.Button class.  Namespace mapping is what links the XAML namespace “xmlns=http://schemas.microsoft.com/winfx/2006/xaml/presentation” in your XAML file with the real .NET namespaces that contain the .NET types.    This mapping is accomplished via the XmlnsDefinitionAttribute class.  At some point in the Silverlight development cycle a programmer at Microsoft decorated the System.Windows assembly with the XmlnsDefinitionAttribute.  Because of that mapping you have access to over a dozen .NET namepaces in your XAML file.
from Walt Ritscher          more here