One of the most interesting pesentations at PodCampWest this weekend was Erik Schwartz who demo-ed Foneshow (blog). This service allows you to publish your audio podcast to a cell phone. Listeners can subscribe to podcasts from the Foneshow website or from the podcaster's website. The listener receives a text message with a phone number in it that the listener dials in order to hear the show. One interesting aspect of Foneshow is that they can tell you how many downloads you get, AND they can also tell you how many people play the show, and for how long they listen. This is pretty cool, and solves that problem of figuring out "who's listening?" question. (Well, technically, I guess all we know is that the cell phone played the episode... whether there's a human actually listening, is not known.) When the episode is played, it's played via the voice line on the cell phone, not the data line. Fundamentally, they have built a voicemail system, so you can do things like forward the episode to another phone number.
On the advertising front, you can have your advertising in the podcast, and then also report back to the advertiser about numbers of listens. You can also add advertising for the cell phone-only shows, so you can have different advertisers for your download podcast vs. your cell phone podcast. (This also solves the problem of the "but I don't have an iPod").
One application Erik mentioned is that you can use the same service/technolgoy to create micro communities around these podcasts distributed through the cell phone. For example, you can create a show and just publish it to your family.