As with anything you see the success stories often because they are interesting which masks the fact that the vast majority of those developing mobile games have trouble just breaking even.

Without anything first hand experience I get the feeling that advertisers are paying to much at the moment for ad space in iphone apps, given that it's a relatively new advertising direction I don't think it's really stabilized yet and going forward it will probably become harder to pull the same kinds of number from that same traffic.