I spoke to Matt May about this in person at the last Seattle blogger meetup and he has written an excellent entry detailing what it really costs to play RIAA music inside podcasts:

Once you have paid the $750 minimum to the performing rights organizations, $42.50 per track to Harry Fox, and whatever is behind Door Number 3 to the labels, you’re covered, at least until more than 500 people download any given song.

500 downloads aren’t very much when you stop and think about the massive size of the web. Creative Commons music is clearly the way to go or produce/create your own music or mix royalty-free tracks using ACID or other tools. It’s depressing, but true, that if you want to legally play an RIAA-encumbered song inside your podcast it is a mighty expensive proposition. The really sad part is that the artists themselves aren’t making much money from all these expenses.