While the headline figure from Distimo's presentation deck from the Verizon Developers Conference was that 72 percent of revenue for iPhone apps comes from in-app purchases, that's not the whole story.

More striking is only 4 percent of iPhone apps include IAP.

The figure seemed so low, we had to check with Distimo that it was correct.

It is.

Freemium cash-in

The reasons are partly due to legacy issues, as IAP was enabled in June 2009, almost a year after the App Store launched.

The other is that although IAP works well for certain categories - notably games - the upfront paid model (or purely free) remains the most popular business model. Obviously, although games are the single largest App Store category, overall it's a minority.

Still, IAP is more popular for certain types of content.

According to Distimo's analysis, as expected, the proportion of free games that include IAP is significantly higher at 11 percent.

It's a similar situation with iPad apps; 7 percent of dedicated iPad content includes IAP, rising to 20 percent for free games

But, the conclusion remains that a mere 4 percent of iPhone content on the App Store is generating 72 percent of the revenue.

And that's why you should be including IAP in your games.