Arduino to iPhone interface cable: Redpark TTL Cable for iOS.
It’s just a cable but … imagine using the Arduino to sense the world around it or to control servos and lights, all remotely. That’s what you can do when you interface microcontrollers to your phone. (For the non-techie, a microcontroller is like a tiny self contained computer on a chip, inexpensive, and capable of interfacing with sensors and sending signals to control the world of real stuff.)
I spoke with a guy two years ago who was using early Android phones to provide the communications link for a remote sensing application. Very cool.