Kevin has a great summary of a new podcast advertising network from Podbridge that includes distribution on the BBC and Clear Channel radio.
Podbridge will serve and track audio ads to podcasts in its network. Most ads will be :15 or less, and can be placed within the podcast, or inserted before or after the podcast. If a publisher wishes to insert ads within the content, he can either use a cue sheet to notify Podbridge of the timestamp where the ad should be, or he can insert a tone into the broadcast which the Podbridge software will recognize and insert ads automatically at that point.
The ads are inserted dynamically and are sold on a CPM basis.