Jeremy Wagstaff has a very insightful column in the Wall St. Journal on how the web has made available cheap, easy-to-use publishing tools which are caused a huge upswing in the number of people sharing their knowledge. If I know something or have an opinion--why not share it?
Link: WSJ.com - Loose Wire (subscription required)
I suspect there's another factor at play in podcasting's popularity. Users are beginning to look at the Internet and technology a little differently. Blogs -- online journals, or Web logs -- have made it really, really easy to build impressive looking Web sites with little or no knowledge of HTML, the fiddly code behind a Web page. Digital photography and video cameras have gotten cheaper, easier to use, and, perhaps most importantly, more closely tied to the Internet, so posting a photograph from, say, your cellphone to a Web page doesn't involve cables, complicated formatting or a degree in computer science.
All this multimedia dexterity has helped blur the distinction between consumer and producer: Now anybody who wants to can be a publisher. This is having another important effect. Before, during the dot.com boom, everyone was looking for the money. Everyone was thinking big scale. Nowadays, not everyone thinks like that. The buzzword du jour is "the long tail," shorthand for the idea that not every business needs to worry about finding a huge audience for its product, (the fat end of the long tail) but instead could find success in catering to smaller, more specialized or localized chunks of the audience (the thin end of the long tail).