Each of the podcasters listed in the Podcast.net directory have the ability to provide a series of keywords that help to describe what their podcast is about.
Tagging is useful when you want to add a meta information to non-text data (URL in del.icio.us, photo in flickr), and add tags to contents not by you. Podcast.net tags is a nice idea in terms of that you can add a tag to audio stream (non-text data), but I think there're siginificant things left to be done:
- Should be able to define tags for each stream, not based on a cast channel
- Should be able to tag other users podcasting with your own feelings
Those ones are important perspective of tagging to multimedia content IMHO.