Collections of objects in mongodb entities/documents should be handled like in the ORM through the Common collections.
This makes it possible to use similar transparent lazy-loading strategies, without $em->loadEntityAssociations which is contrary to the idea of transparent persistence.
Also this would make mongo entities/documents compatible with orm/relational entities, that is, using the same class(es) as a mongo and orm entity.
This goes for other potential upcoming projects, like couchdb, too. We should always encourage the use of the Doctrine Common collections in domain models.