Billed as "AdSense High-End", Google is now letting advertisers create ads specifically for "high-end" phones like the iPhone and Android-capable devices. While that is a natural evolution of the AdSense platform, Google's explanation as to why is quite interesting:

With the growth in popularity of high-end mobile phones with full (HTML) Internet browsers, like the iPhone and phones running Android, advertisers are increasingly extending their campaigns to mobile devices. Because these devices offer a rich content experience similar to desktop, advertising on them is a natural extension for many advertisers.

... In other words, the mobile web is really the traditional web. Experiences and applications are becoming richer - and will continue to do so as devices and HTML / HTML5 allows for it. And therefore, the advertising experience will also become richer and more effective.