What started as a lark, playing around with an operating system that would allow Doom creator John Carmack to quickly produce portables games, has become a thriving business, the famed developer tells Kotaku.

"Wolfenstein Classic was my original experiment on whether a first-person shooter would be any fun on the iPhone," he said. "It did surprisingly well for all of us."

So well, in fact, that Carmack finds himself spending a disproportionate amount of his time working on future iPhone games. Already id Software has released Wolfenstein Classic, Quake RPG, Doom Resurrection and this week Doom Classic.

Carmack said that there was a lot of "hand wringing" initially over the idea of spending the company's own money (there was no publisher to help fund development) on making games for the iPhone. Doom Resurrection, when it hit, was probably the most expensive game to develop for the iPhone, Carmack says.

But that internal concern quickly disappeared when Wolfenstein Classic hit the App Store.

"It did really well for us," he said. "It was Wolfenstein Classic tha