What is a podcast?

Mike Arrington recently reignited some discussion about the definition of a blog.

I have a related question: What is a podcast?

I'm really only concerned with two technical questions:

1. Does each feed item need an enclosure? If not, what ratio of items with enclosures to total items makes a feed a podcast?
2. Does the enclosure need to be in a dedicated feed element, or is it okay to just put a link to the enclosure somewhere in the feed description? For example, is this a podcast? The publisher, a big RSS technology company, seems to think so.

There are other debatable points, too, such as whether Public Radio feeds are really podcasts, but I'm more interested in the technical points.