You can determine the outer bounds of the elements you want to display and set that to the window rect. For example, with the states sample if I call this on imported:

private void usaLayer_Imported(object sender, MapLayerImportEventArgs e)
        {
            double minY = double.MaxValue;
            double maxY = double.MinValue;
            double minX = double.MaxValue;
            double maxX = double.MinValue;

            foreach (MapElement ele in (sender as MapLayer).Elements)
            {
                if (ele.Name == "Iowa" || ele.Name == "Indiana")
                {
                    minY = Math.Min(minY, ele.WorldRect.Top);
                    maxY = Math.Max(maxY, ele.WorldRect.Bottom);
                    minX = Math.Min(minX, ele.WorldRect.Left);
                    maxX = Math.Max(maxX, ele.WorldRect.Right);
                }
            }

            if ((sender as MapLayer).Elements.Count > 0)
            {
                theMap.WindowRect = new Rect(minX, minY, maxX - minX, maxY - minY);
            }
        }

It will snap the screen to contain Iowa and Indiana.

-Graham