Podcasting is a mainstream medium in the making according to Bridge Ratings' "Future of Podcasting" study of radio listeners in 10 national markets, writes eMarketer. The study estimates that so far this year some 4.8 million persons have downloaded a podcast from a radio station or other source - and that by year's end some five million people will have downloaded podcasts, compared with only 820,000 in 2004. Bridge Ratings also forecasts that the podcast audience will reach critical mass in 2010 with 62.8 million users.

Bridge foresees a rapid increase in podcast use, from an estimated 9.3 million in 2006 to 20.9 million in 2007, and nearly double that in 2009 with 40.5 million.

The Bridge study showed that approximately 20 percent of users who have downloaded and listened to a podcast have done so on a weekly basis - an average of six podcasts per week - and have spent some four hours a month listening to podcasts.