Now, if you browse iTune's Top Grossing applications, you will notice that they also display the most popular In App Purchases. The example below is from Electronic Art's Rock Band, the highest grossing app on iTunes. The game itself is relatively new (notice just ~200 user reviews) and costs $9.99 per download. Just below the purchase button though is a banner to view "Top In App Purchases" - which displays the highest grossing $0.99 Rock Band tracks.
While this is a small example (Rock Band itself is new enough that the purchasing habits / offerings are light), but it is clear that Apple is trying to promote the same behaviors that are becoming so popular - and profitable - on Facebook: in-app / in-game micro transactions.