Apple plans to add advertising in a very selective way to iTunes, AdAge says. The ads will appear in the "lower-left corner of the iTunes library while users listen to podcasts from their computers rather than from portable devices," the publication writes. (Via Steve Rubel).
Apple reportedly is being careful about adding advertising, but I bet this is only stage 1 of a bigger rollout of advertising and for-fee podcasting services. Apple has the ability by aggregating all of that content to track usage, dynamically insert ads, and figure out which ads have been listened to. Maybe they won't go that far. They certainly don't need to.
But at the very least, they could sell podcasts, and they have the structure in place within iTunes to do that. As Rubel pointed out a while ago, within iTunes there is already a little button next to a podcast listing whether it's a free podcast or not.