Joining the podcast audience requires confident use of an iPod or MP3 player, as well as ease with the process of downloading syndicated digital content from the internet (although podcasts can be listened to and watched on a computer with an internet connection, it's their portability that attracts users). This will remain a constraint on audience size even though the number of consumers who meet these requirements will inevitably grow over the next few years.
If we expect that the distribution mechanism for podcasts in 2010 will still involve the cumbersome process of downloading to one's computer and placing it on one's portable device, then we're probably being short-sighted. Personally, I think podcasting will explode and the audience will be much larger than 15 million people. Of course, by then, it probably won't be called podcasting anymore because there will be multiple distribution mechanisms and we'll be able to get our favorite programming without having to connect our portables to our PCs. How does on-demand IP content sound? I think it will get here before 2010.
That's the problem with these kinds of projections. They assume that the distribution mechanism will remain constant. Guess what? If the demand for the content is significant, we'll invent new ways to get it out to people.