Here's an interesting little finding regarding Apple's iOS devices -- Mobclix claims that for the month of July 2010, ads on the iPad were actually more lucrative than ads running on iPhone apps. The effective cost per thousand impressions is actually five times higher on the iPad than on the iPhone, which means that because there are fewer iPads around (but presumably ad rates are currently around even for the two devices), advertisers are actually paying much more to reach the same amount of people on the larger device as the smaller one.
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.