It's a shame it seems to be network ads (so far). I can't imagine they'll pay too well. It's a lot of effort to self-sell ads but it's rewarding, too. People using Overcast are interested in podcasts, and I'd bet there are hundreds of podcasts that would pay per subscriber they could obtain via ideal positioning like this. (Or even just paying to be shown as a "similar" podcast to others the user subscribes to.)
(Heck, let's start the bidding.. I'd pay $1 a subscriber for a podcast I have in the works and I know there are people & companies with far bigger plans and deeper pockets than me.)