That's what Advertising Age's sources are tell them. It
seems 'Apple's current plans call for the ads to appear only in the lower-left corner of the iTunes library while users
listen to podcasts from their computers rather than from portable devices'.
Hmm, what in the lower left hand corner of the iTunes library... why it is the little window that displays album art, and any graphics that might be embedded in an 'enhanced podcast.' Sounds to me like Apple is recommending podcasts use AAC's for their podcasts so they can embed graphics and links, though that wouldn't make Apple any money.
Let's hope Apple won't be turning our copies of iTunes into a billboard anytime soon.