AdMob recently announced their statistics that 54% of iPhone applications have showed less than one thousand ads and only 1% have showed over 1M ads.
At mobile ads click thru percentages are normal (mine has been around 6%) compared to Google Adsense (lower than 1%).
And now Google is trying to get to the market with gPhone. Besides being a technical fiasco, it could be financial disaster for developers as advertisement based business model makes profits for Google but not to developers. And who would use Adsense for mobiles where Google has been taken 80% commission where AdMob only takes 30%?
But the bottom line is that no-one bothers to click on those Adsense ads. Google does not have interesting advertisers there.