Thanks Graham. I ended up going in a different direction but that info is very helpful and will probably be used in a different project.

Maybe you've figured it out, but there are all sorts of things you can do with the element properties to help you partition items in a layer. And the element properties can be specified by you or come from your shapefile. Here's an example:

    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }

        private void statesLayer_Imported(
            object sender,
            Infragistics.Silverlight.Map.MapLayerImportEventArgs e)
        {
            foreach (MapElement ele in
                (sender as MapLayer).Elements)
            {
                if (ele.Caption[0] < 'M')
                {
                    ele.SetProperty("Special", true);
                }
                else
                {
                    ele.SetProperty("Special", false);
                }
            }
        }

        private void theMap_ElementClick(
            object sender, 
            MapElementClickEventArgs e)
        {
            if ((bool)e.Element.GetProperty("Special"))
            {
                foreach (MapElement ele in 
                    (sender as XamWebMap).Layers[0].Elements)
                {
                    if ((bool)ele.GetProperty("Special"))
                    {
                        ele.Fill = new SolidColorBrush(Colors.Red);
                    }
                    else
                    {
                        ele.Fill = new SolidColorBrush(Colors.Blue);
                    }
                }
            }
            else
            {
                foreach (MapElement ele in
                    (sender as XamWebMap).Layers[0].Elements)
                {
                    if (!(bool)ele.GetProperty("Special"))
                    {
                        ele.Fill = new SolidColorBrush(Colors.Red);
                    }
                    else
                    {
                        ele.Fill = new SolidColorBrush(Colors.Blue);
                    }
                }
            }
        }
    }

-Graham

Sort of. Can you give a quick example in vb or c#?