It appears there are no built in function the API that does this, so this is what I came up with. Please note the special case where the bounding box overlaps the 180th degree longitude.

/**
* Returns true if point is in rectangle.
*
* @param {VELatLong} point Point to check.
* @param {VELatLongRectangle} rect Bounding box.
*/
function isPointInRect(point, rect) {
    return ( (rect.TopLeftLatLong.Longitude <= rect.BottomRightLatLong.Longitude && // longitude
              rect.TopLeftLatLong.Longitude <= point.Longitude &&
              rect.BottomRightLatLong.Longitude >= point.Longitude)
           || (rect.TopLeftLatLong.Longitude > rect.BottomRightLatLong.Longitude &&  // longitude crosses 180 degrees
              rect.TopLeftLatLong.Longitude >= point.Longitude &&
              rect.BottomRightLatLong.Longitude <= point.Longitude) )
           && (rect.TopLeftLatLong.Latitude >= point.Latitude &&     // latitude
              rect.BottomRightLatLong.Latitude <= point.Latitude);
}

This may be used, among many things, to test if a pushpin shape is within the current map view, and if it isn’t the map is panned to make it visible:

var point = shape.GetPoints()[0];
if (!isPointInRect(point, map.GetMapView())) {
    map.PanToLatLong(point);
}