The ability for developers to deliver free apps with in-app purchasing always seemed like a good model to me. Flurry took a reasonable investigation model (looking at the top 100 grossing games from January to June 2011) and derived interesting results that supports what had just been an opinion on part supported by anecdotal information.
In the relatively short recent time span, Flurry found that the model had flip-flopped with respect paid versus freemium apps. In January 61% of the revenue for the top 100 grossing games came from premium (paid) apps. By June, freemium apps accounted for 65% of revenue with premium apps drawing in the remaining 35%.
Flurry notes that freemium apps have the potential to generate more revenue that paid apps because extremely engaged players may pay much more than they would have for a fixed-priced paid app.
It will be interesting to see periodic updates from Flurry on this topic. It may be, for example, that there is a surge in buying paid apps early in the calendar year because many people get iTunes gift cards which they use to pay for apps they may not have normally considered. It may also be that people who received a new iPhone or iPad in December are more likely to buy apps for month or two following the device acquisition.