Thomas Crampton, a guest poster on Joi Ito's Wep points to an interesting Wired article on how few podcasters are able to maintain their effort over an extended time:
Joi Ito's Web: Podcasting: The end of amateur hour?
Interestingly enough Thomas' presence as a guest blogger is a partial solution to the problem of keeping a periodic publication active in the face of declining interest by the founder who was once one of the most popular bloggers but whose "ranking" has been eclipsed by blogs which talk about gadgets and the oddities of the world.
Personally I don't see the emergence of the "mainstream media" in what Thomas calls the "new media space: as a threat to any one who is doing what they like to do. Unless what they like to do is be a big star...
Of course, what do I know? I am not a podcast listener.