Recently I recieved this wonderful piece of news when I ran ./manage.py dumpdata for the first time:
Error: Unable to serialize database: User matching query does not exist.
I knew this might not work out since I was dealing with a legacy database, but the resolution is quite simple.  First I had to narrow it down to which app was causing this.  Naturally I assumed it was one of the two apps I had, either common or restaurant.  So I ran: ./manage.py dumpdata common and ./manage.py dumpdata restaurant.  The latter had no problem whatsoever.
This made sense, since my common application was the only one that made any reference to a User.  By looking in my models.py for that application, I narrowed it down to my Profile object.  Sure enough, commenting it out meant I could get my data.
It ended up being a foreign key mismatch between the profile and user tables.  Since this is legacy data, this mismatch made sense.  A simple SELECT id,userid FROM profile WHERE userid NOT IN (SELECT id FROM auth_user) gave me a list of bad profiles.  Removing them allowed me to create my Django fixtures.