The idea is that postgres would be a storage engine for mongodb, like innodb is a storage engine for mysql
I don't think MongoDB would have much left to do, if PostgreSQL was doing the storage duties. The converse - implementing the MongoDB protocol within PostgreSQL, as mentioned above (mongolike), sounds like a more sensible proposal.
threeseed probably wants to use Postgres as a robust transactional backend for MongoDB much like InnoDB is used as a robust transactional engine for MySQL. (There is TokuMX that might fit that use case also).