This example uses a light sensitive resistor that is attached to an iPhone screen. A simple application adjusts the brightness of the area of the screen under the sensor and this is detected by the Arduino.
This example shows how an iPad application can act as a remote control for a Toy robot using the MIDI protocol.
The excellent PGMidi library from Pete Goodliffe: https://github.com/petegoodliffe/PGMidi
This example shows how to control a mobile robot using an on-board iPod touch and the Redpark serial cable.