I recently put a book proposal together and have therefore had my head in podcast stats. The one thing that's pretty clear is that nobody has a freakin' clue how many people listen to podcasts.
The last (or at least most commonly quoted) study done recently by Pew Internet & American Life Project pegs listenership at 6 million. Another study by eMarketer put it at 10 million in 2006. Today's number from Forrester says only 1% of North Americans regulary listen, but 25% are "interested" in podcasts.
What's the right number? I don't think it's worth thinking about yet. Forrester's 1% of North Americans is about 3.3 million people (I'm going to assume they mean US and Canada only). So we're looking at a range from 3.3 to 10 million. In the scope of a 330,000,000 potential market, that's a spread of a shade over 2%. The margin for error on these studies is likely much higher than that and therefore I don't think these studies are worth getting excited about. When we start seeing numbers like 50 million, THEN I'll get excited.
The one stat in these studies that rings false to me is Forrester's claim that 25% of North Americans are interested in podcasting. I call foul on that. I challenge anyone in any city in North America to walk down the street and find four people who even know what podcasting is inside of an hour. I operate in this space daily and 25% of the people I associate with don't even know what podcasting is. Heck, a lot of them don't even really understand what a blog is.
Before I started making business decisions based on podcast stats, I'd want to see some significant and consistent numbers.