This example demonstrates the use of GeoJSON over Bing tiles.  The
        GeoJSON vector data is in geographic coordinates (EPSG:4326).  The Bing
        tiles are in a spherical mercator projection (EPSG:900913).  By setting
        the projection property of the GeoJSON layer to the source
        projection (EPSG:4326), the features are properly displayed over the
        base layer.  In general, the map projection determines how raster or
        vector data is displayed.  The layer projection corresponds to the
        projection of the data source.
        
See the geojson-reprojected.js source for details on how this is done.