I don't know what the radio numbers are, but I do know that in my neck of the woods getting a 1% return rate on a flyer is pretty damn good.
While I will agree that there's a chance some podcasts that are downloaded don't get listened to, I think that's really the exception and not the norm. Unlike radio, a a podcast takes some positive action on the listener's part. It's not push media - it has to be pulled. And while it's likely that some of us don't unsubscribe from shows as quickly as we might when we've decided not to listen to them any more, it's a safe bet that almost all downloaded podcasts are listened to.
Podtrac has just released a survey that states that 88% of their listeners listen to each entire show. That's lower than I thought, but one hell of a lot higher than the 1% of people that read my flyers. Or the 0% that actually hear any of the talk or ads on the radio.
Podcast advertising is powerful. It can not only target listener's interests better than anything main stream medai has to offer, listeners also actually hear it.