Google has launched Adsense for Radio through its dMarc acquisition. They are already running ads on a Detroit radio station plan to make the service "generally available" within the next three months. Google also announced a deal with XM Satellite Radio today to automatically insert ads on non-music channels.
Google Adsense has been so successful not because of the big name sites but because of what Chris Anderson would call the Long Tail of Advertising. I know that many podcast producers are still trying to figure out how exactly to monetize the podcasts and subsidize bandwidth costs. Google Adsense would do for podcasts what it did for the web - enable an entirely new market of small-time, "long-tail" producers in niche markets.
Of course, it's not quite as easy with podcasts as they are generally downloaded as opposed to streamed. This raises real logistical problems when trying to figure out when and where ad insertion happens and how to accurately track the listeners. It will be interesting to see how they will handle payment on the radio ads and if they will apply that model for podcasts since the subscribers are on a much smaller scale.
That said, it's really just a matter of time...