Marcia over at the Search Engine Watch Forums started a thread named What percentage of ODP submissions are made by corrupt webmasters? In that thread they get into the discussion of what type of directory spam they find and where spam is more likely to be submitted to.
The most obvious are categories "where there's tons of money to be made", such as m mortgages and poker. They say ">98% slammed by lead-generating sites" for those money categories.
But they also say its not always that easy to know which sites are submitted with the intent of spamming versus those that submit without really knowing the rules and guidelines.