I checked out the three podcast directory and I liked Podcast.net because you could narrow the categories down a bit further there than on the other two. Instead of just sports you could find podcasts about types of sports under that heading. That is much more helpful. I'll have to mess around with that some more at home and see if I can find some to add to my Ipod.
I also found this nice column from PC Magazine about podcasting the death of radio.