AdAge has an excellent article that looks at the “Big Three” companies trying to hit podcast advertising head on - Podshow, Podtrac, and PodBridge - as well as the difference in opinion regarding who - and how many - are actually listening to podcasts. Podshow was the source of a lot of controversy from the podcasters of the world when the Podshow contract was anonymously emailed to the podcast Keith and the Girl - which they read online.  To date, neither Podtrac or PodBridge have generated much heat from podcasting discussions but more podcasts (including some of the favorites I reviewed back in 2005) have been including ads during their podcasts.