Dear Bo ,
Message driven bean implements messagedriven listener .So once a message drivenbean is deployed with the destination Queue or Topic mentioned in the deployment descriptor it will always listen to the destination and once if the destination receive any message it will be consumed by the MDB.
This is asynchronous receiving.unless in the case of other enterprise beans only if any client invokes the bean it can receive the message from the destiation.But in MDB, container will invoke whenever needed.