If you think about how mobile phones will be used, then I think we can get an idea of where and how mobile ads will and should come into play.
Local Search- Mobile users will use their phones to search for places to eat, shop, and be entertained. In these cases, targeted ads appearing with search results would be based on keywords and location.
Opt-in promotions- users could opt-in to special offers based on their interests and location/proximity. Offers would pop up in SMS-style alerts depending on proximity to participating advertisers.
Mobile Web- while browsing mobile sites on a mobile device, click-through ads like AdMob ads will appear on the mobile site. I think these will be rejected unless they are unobtrusive, targeted text-only ads that take up very little screen real estate.
I think local search and opt-in advertising will be the most lucrative on mobile devices because people will use their phones to search for businesses and services that they will end up visiting in person to spend $$. Why do you think Google has partnered up with the likes of
Palm on the Treo 700W,
Motorola, and
RIM/Blackberry to get a Google search box front and center on their handsets?
I have a hard time believing that people will use their mobile devices to buy products that will be delivered at a later date/location. Ringtones were mentioned, and while they are a huge market, most users will get sick of seing ringtone ads everywhere on mobile sites.
I highly recommend the book
Going Wireless by Jaclyn Easton to anyone interested in the future of mobile advertising and wireless devices in general. She dedicates a significant portion of the book to mobile advertising/marketing and location-based services.