There is no CDN URL display on the container details page at http://example.com/admin/config/services/hpcloud-object/container/XXX, it always displays empty. It seems that the $container variable may not be initialized properly. If I start with a scheme that's assigned to a container, say:

if ($wrapper = file_stream_wrapper_get_instance_by_uri('hpcloud://')) {
  print $wrapper->container()->cdnUrl();
}

then it prints all right.