Kottke: "Podcasters have been slower to break out of the mold provided by talk radio. The playing of music before segments and as transitions between segments makes some sense on the radio, where it's used in some cases to fill airtime. But for podcasts, there's no need to fill airtime with anything but content. 30 seconds of music before the actual podcast begins is the audio equivalent of Flash splash pages on web sites."
I totally agree with this. The Onion Radio News is probably the best example of built-for-the-web audio I know of (and it even has ads). Of course, a lot of podcasters still produce the iPod, rather than the web.