But that cost is paying off -- iPad users tend to spend about three times as much time playing with apps on their devices versus iPhone users of the same apps. Of course, given the brevity of a typical iPhone usage profile, that's still only about ten minutes more per app on the iPad. But it's quite clear already that the iPad presents a different use case, and that advertisers will have to consider the two almost completely different audiences.
Keep in mind that the iPad is still in its relative infancy -- the iPhone has only been around for a few years, but we're still only talking about a few months (and even less than that, considering these numbers come from July) on the iPad. It'll be interesting to see how these two devices continue to diverge in the future.