I just tried to create a custom info window for a Google Map using the Google Maps API For Flash. Unfortunately it does not seem to work as expected by setting the customContent property in an object of type InfoWindowOptions. Therefore I wrote this little workaround that creates a popup window:

private function createInfoWindow(map:Map, marker:Marker, infoWindow:Class):IFlexDisplayObject {
  var popup:IFlexDisplayObject = PopUpManager.createPopUp(map, infoWindow);
  var globalPos:Point = map.localToGlobal(map.fromLatLngToViewport(marker.getLatLng()));
  popup.move(globalPos.x - popup.width/2, globalPos.y - popup.height - marker.getDisplayObject().height);
  return popup;