The number of people who listen to podcasts is growing, according to data collected by eMarketer. The total US podcast audience reached 18.5 million in 2007 and is projected to continue to expand exponentially for the foreseeable future.
See all the data at http://www.emarketer.com/Article.aspx?id=1005869
While all this sounds promising, before you invest any of your resources in podcasting, I recommend that you do some research about the specific audience(s) you want to reach. Numerous studies and anecdotal data indicate that podcast use is still largely concentrated among 20-somethings.