Since we do our own Direct3D rendering we can get better performance with polygons and polylines on our map by managing the vertices ourselves and just adjusting the camera per frame instead of pushing across new vertices. As a side effect of this MapPolygon and MapPolyline do not descend from UIElement and can't be placed just anywhere in the UI tree but instead need to be children of the separate Map.ShapeLayers property to differentiate them from XAML overlay controls. XAML children still go on the Children property like you would expect.
<bm:Map x:Name="map" Width="800" Height="600" Credentials="INSERT_YOUR_BING_MAPS_KEY">
<!-- UIElements go under Map.Children -->
<bm:Location Latitude="0" Longitude="0" />
<!-- MapPolygon and MapPolyline go under Map.ShapeLayers -->
<bm:Location Latitude="-10" Longitude="-10" />
<bm:Location Latitude="-10" Longitude="10" />
<bm:Location Latitude="10" Longitude="10" />
<bm:Location Latitude="10" Longitude="-10" />
Side note: there's a XAML compiler bug in the consumer preview that prevents setting FillColor on MapPolygon but you can set it via code fine or wait until the final release where it can be set from XAML as well.
Side note 2: We're already hard at work creating the final version of the Map control with tons of tweaks and fixes. As an example Pushpin no longer requires setting PositionAnchor on it as it will default to the center of the pushpin.