Array
(
    [_id] => MongoId Object
        (
            [$id] => 4e465de048177e8105000003
        )

    [id_usuario] => MongoId Object
        (
            [$id] => 4e43b20648177e5305000000
        )

    [mensaje] => lero lero si?
    [created_at] => MongoDate Object
        (
            [sec] => 1313234400
            [usec] => 160000
        )
    [comentarios] => Array
        (
            [0] => Array
            (
                [comentario] => hola mundo
                [usuario] => [_id] => MongoId Object
                    (
                        [$id] => 4e465de048177e8105000003
                    )
                [created_at] => MongoDate Object
                (
                    [sec] => 1313234400
                    [usec] => 160000
                )
            )
            [1] => Array
            (
                [comentario] => hola mundo
                [usuario] => [_id] => MongoId Object
                    (
                        [$id] => 4e465de048177e8105000003
                    )
                [created_at] => MongoDate Object
                (
                    [sec] => 1313234400
                    [usec] => 160000
                )
            )
        )
)

$db->usuarios->find()->sort(array('created_at' => -1))->limit(10)->skip(0);

$db->usuarios->find()->sort(array('created_at' => -1))->limit(10)->skip(0);

$user = $db->usuarios->find()->sort(array('commentarios.created_at' => -1)->limit(3);
$comments = array_merge($user[0]['commentarios'], $user[1]['commentarios'], $user[2]['commentarios']
$ordered = uasort($comments, 'sorter')

function sorter($a, $b) {
  return $a['created_date'] < $b['created_date'];
}