How about the less-sensational: "One Third of Top-Grossing iPhone Apps Profit With In-App Purchases"
If all you're trying to do is to get someone to upgrade from lite to full, that can only happen once. In-app purchases can happen several times. In Restaurant Story (one of the games mentioned in the article), you buy gems to speed up the process with which you make food. I'd bet that most people who purchase those gems do it more than once, because they're directly tied in to your success at that game. In short, there's the potential here for recurring revenue.
Not only that, but the prices on in-app purchases can be startling! Going back to Restaurant Story, I think the smallest batch of gems you can buy is $5 and it goes all the way to $99. That's a lot more revenue per user, potentially, than on an upgrade from lite to full.
I think it's pretty easy to make the connection that Free Apps who also have revenue are selling things within the app.
Also, the article makes clear, it's not just people paying for the "full versions," but for virtual currencies, select features, etc.