Google launched a beta AdSense for Mobile Applications allows you to monetize your mobile applications by showing contextually targeted ads and/or placement targeted ads alongside your application content on iPhone and Android devices. The iPhone SDK is compatible with iPhone OS 3.0, and Android SDK is compatible with Android 1.5 SDK. You can show 320×50 text and image ads linked to HTML webpages in your application. These ads are targeted to the keywords that you send us in the AdSense (or DoubleClick) ad request. The keywords must be relevant to your application content. If your application content is loaded from a webpage that is customized for iPhones and Android handsets, then you can also send us the webpage URL for us to target ads. The ads may also be placement targeted which means an advertiser can specifically target to your application. You can sign up here.
More info: Code Blog