The shift from straight paid games to in-app purchases, freemium or ads (or a combination of all three) on Apple iOS devices, has taken longer than was initailly expected.

Over a year on from the release of the iPhone OS 3.0 firmware that enabled such business models, developers are finally throwing their full weight behind the option.

But while large publishers such as ngmoco and Glu Mobile have made the headlines, smaller indies have also radically shifted their ways of doing business.

One such is highly regarded US outfit NimbleBit.

It's always been open about its App Store successes and failures, and as co-founder Ian Marsh explains, the release of its new freemium game Pocket Frogs demonstrates that it believes free games can be profitable; indeed more profitable than paid games.

How does the business model work for Pocket Frogs?

Pocket Frogs is free to play with optional in-app purchases that speed up gameplay. All of the content in the game can be accessed whether you use the IAP or not.

Stamps can be earned in the game (or purchased), which instantly deliver frogs and other items you order in game or find in the pond. Potions can be earned in the game (or purchased), which instantly mature a frog. There are also some iAds in unobtrusive parts of the game.

You've recently said NimbleBits isn't making any more paid games. Why?

Scoops has taught us that free games can be profitable so it becomes a question of whether you want to make a living with thousands of people playing your games or millions.

We put so much time and effort into our games that we'd like as many people as possible to enjoy them, which is part of what I love about free to play games.

Sky Burger has done a million free downloads so do you think you can successfully cross-promote that audience to Pocket Frogs?

That is the hope. We've learned over the past two years that having a large audience can be a powerful thing.

Every free giveaway we've held for our games has ended up being worth more in the end than any lost revenue.

What do you think is the biggest change in your business over the past 12 months?

Obviously we are investing a lot into the idea of free to play and trying to leverage our existing audience. Pocket Frogs will be the first true test of this new model for us, so a lot is riding on the game.

Are you considering developing on any other mobile platforms?

No. We remain dedicated solely to iOS for the time being. We've found a comfortable place on the App Store and I certainly don't have the time to develop for any other platforms without contracting out the porting work which I'm averse to.

Coming from traditional cell phone development, I know what it is like to try and target a wide range of different devices and I hope to never have to do it again.

Thanks to Ian for his time.

Pocket Frogs is released as a universal app for iPhone and iPad on September 16.