Knowledge@Wharton has an interview with Omar Hamoui of AdMob. Omar’s response to how mobile advertising is different from web advertising:
The targeting is [also] very different. If you are an advertiser, you likely care what carrier the person is on, what handset they are using, whether the handset will support Java or [the] Symbian [operating system]; is it Microsoft Windows or is it Blackberry? — all those things matter to you. Online, none of those things matter.
The web pages themselves are not translatable onto mobile [devices]. You have to have a different landing page. We would love it if we could just find an ad network to fill our inventory with existing online advertising. But we can’t, because those ads would link to web pages which wouldn’t render on the mobile device in the first place. [The mobile web] is sort of its own silo.