I agree with nearly all of Rebecca's points, except for one (well, maybe two...). The first is "Podcasts offer zero interactivity".
Podcasting can absolutely be interactive. As a producer, you have to engage people and invite them to interact. Evidence of interactivity can be found in things like forum participation, weblog comments, listener emails, etc.
Response mechanisms for specific messages or offers can be integrated into these feedback mechanisms. No, it's not like other forms of Internet advertising, and you shouldn't expect it to be. But, if your audience is making the leap from an audio file to the web to engage YOU, one can assume that your audience is truly engaged, self-selected and receptive to a relevant marketing message.
A qualified audience can only be a Good Thing.
Additionally, if the "MP3 Player is Optional" as Rebecca posits (Which is likely true. Certain research pegs PC listenership of podcasts at around 50%) then the opportunity for interaction is even greater, since your listener is sitting in front of device that allows for interactivity.
As for the second point to which I partly disagree, "Podcasting offers iffy metrics," my best response would be to say, "Yes, but...". (Warning: here comes a plug, but it's my blog, m'kay?)
AudibleWordcast (Pro) is the only podcasting platform that has been determined "audit-ready" for reach and frequency by a credible third-party media auditor. So, you get real metrics tied to bona fide human beings, not just downloads.
No, it doesn't use mp3, but the Audible Audio format is the de facto standard for spoken word audio, playable on more than 200 devices, including every iPod ever made. It's the most widely playable file format on portable devices, second only to MP3.
OK, enough plugging.
Go read the articles. They're relatively short, informative and worth your time.