Infinity Field HD is the first app to include multiplayer Joypad support, allowing two players to play the game in co-op multiplayer mode on one iPad while controlling it from two iPhones, as shown in this video:
iOS developers have been pushing the boundaries of what's possible on Apple's gaming platform. Last year, Firemint used AirPlay to project a game on a television via an AppleTV, and used an iPad as a second screen to control the game.
Infinity Field is available for the iPhone or the iPad, for $0.99 or $1.99 respectively on the App Store. Joypad is free, while Joypad Elite is $11.99 and includes a number of skins from classic console controllers. Select skins are available via in-app purchase from the main Joypad app, as well.