From Blogarithms comes this post about the "iPod Economy". He cites this article in Business 2.0 that states there are nearly four million iPods out there and Apple has 54% of the MP3 player market. When I was talking about all the pieces being there for an absolute explosion in podcasting, I was shooting low. 4 million of these are in use, and that's just the Apple ones. Moreover, they continue flying off the shelves.