I'm not sure why Google would make much more in Adsense revenue on Android than they would on iOS. I think the primary issue is not conceding control of the platform their revenue is reliant on to a competitor.
iPhone app developers tend to use a variety of ad networks, based on whatever pays best. iAds usually has the best CPM, but inventory can be limited - and the fallback is usually AdSense (or... AdMob, which is also Google).
Aren't iPhone app developers using iAd?