+ Drupal 4.4 stored profile data in the serialized user->data column. Drupal 4.5 stores profile data in tables (but user->data is still available and used for other stuff, like locale or themes).  The update from 4.4 to 4.5 didn't remove the old data from the user->data column properly, because there is no mechanism in user_save to do so (it did try to unset the fields, but this has no effect).

  + On registration, hook_user('insert') is invoked after saving the data column. This means that any module-specific data is put into the data field. We cannot move hook_user('insert') higher up, because before that point, we do not have a complete $user object yet.