Chris and Josh, the guys behing the great podcastweb2.0 show, have created an advertising network for podcasts called Podvine.
While there's nothing extraordinary in an advertising network, Podvine has adopted a brilliant strategy:one ad per show - not more. They believe in the power of one targeted ad compared to dozens of unrelevant ones, and I think it's the right way to go.
At $1600 the two-weeks slot, they probably won't make much money (revenue will be split across the 3 members) but they won't bore their listenners, and it's what really matters when it comes to ads. Podvine could be compared to The Deck ad network, but podcast-focused.
Wish them luck.