Due to a bug in a shell script that tries to ensure journalling is on (I wish you had made that the default for the packaged builds) I ended up with the option twice. Ie my /etc/mongodb.conf ended with

journal = true
journal = true

If you try to start the server "service mongodb start" then it will say started successfully and give a pid. However a ps will show no process. There is also absolutely nothing in /var/log/mongodb/mongodb.log or in any other logfile in /var/log.

Trying to find out why the server didn't start was rather difficult.