Games developers iD Software and Freeverse have both confirmed they're looking into the iPhone SDK.
Both games developers expressed an interest in joining EA and Sega in games development for the device.
Id software head John Carmack said: "The iTunes distribution channel is really a more important aspect than a lot of people understand. The ability to distribute larger applications than the over-the-air limits and effectively market your title with more than a dozen character deck name, combined with the reasonable income split make this look like a very interesting market."
"This type of developer/customer interaction is probably the wave of the future for mobile devices, it will be interesting to see how quickly the other players can react," he added. "Based on our experiences with the carriers, I am betting not very quickly."
Freeverse has confirmed it is already working on a range of games for the device, Golf, Bowling, Soccer, Baseball and motor-racing titles.