According to some market research types, only 840,000 or so in the US regularly listen to podcasts. With all the hype, I would have put the number much higher. However, those same analysts are saying that the total US audience for podcasts will grow to 56 million people in the next five years.
There’s a lot of good stuff being done with the format but I don’t see a huge growth like that without some big changes. For one thing, finding and using the content is still hard work. Apple made it a little easier with the inclusion of a podcast section in iTunes but it still isn’t "mom easy" (no offense, Mom!).
In addition to ease of use, the content itself will also need to change. It must get more mainstream and remain narrow at the same time. There has to be room for the big media companies, which seem to dominate the front page in iTunes, and the quirky, personal recordings that appeal to a community much smaller than the mass marketeers want to address.
But even with easy access and a wide selection of programming, that kind of huge growth in podcasts will never materialize if everyone gets greedy. Trying to charge for distribution or stuffing the programming with ads will turn podcasting into common, ordinary radio, the kind of stuff we’re trying to avoid in the first place.
It seems to me that, barring some unexpected change in copyright and licensing laws regarding music, “professional” podcasting will soon utterly swamp “amateur” podcasting.