I used the J2smf component from GMITC.
I modified the advanced.php to create an smf_member if no corresponding smf_member exists for the registered Joomla! user visiting the forum.
I had to modify the LogInOut.php to know what to do with the Joomla! password and salt.
Now it works like a champ! A registered Joomla! user who logs in and browses to the forums is silently logged in. If no corresponding forum account exists, one is created for the user. If a registered Joomla! user is not logged in and browses to the forums, and logs in through the forum login screen, he is silently logged in to Joomla!