Apple iPod, iPhone (2g, 3g), iPad Dock Device Pinning 

(from: Pinouts.ru)

Here's the pinning of any pin of the socket. Some of them change the way the device interface with another device by the value  of the resistor connected to the pin (This concept is quite important to understand the aim of the lesson).

Testing Comunication between Arduino and iPhone

This explanation is based on some of the wonderful material David Finlay is sharing on its blog, and expecially here. iPodSerial Library is hosted on David's github. We have put a copy of the libraries we are using as attachment to this lesson: iPodSerial and Bounce.

Install both libraries and upload the code to the Arduino-

Important: Arduino won't upload if pin 0 and 1 are still connected to the breakout board. We advice you to find a way to plug and unplug those two pins (we used two female jumpers).

// Simple example of using iPodSerial library to send
// Simple Remote Play command on the press of a button.
// This will alternate between Play and Pause on the iPod.
//
// Uses the Arduino Bounce library for debouncing.
// You can get this from 
//http://arduino.cc/playground/code/Bounce
//

#include <SimpleRemote.h>
#include <Bounce.h>

const byte PLAYPAUSE_PIN = 2;
const byte NEXT_PIN = 3;
const byte PREV_PIN = 4;
const byte TSHUFFLE_PIN = 5;


const unsigned long DEBOUNCE_MS = 20;

Bounce play(PLAYPAUSE_PIN, DEBOUNCE_MS);
Bounce next(NEXT_PIN, DEBOUNCE_MS);
Bounce prev(PREV_PIN, DEBOUNCE_MS);
Bounce tshuf(TSHUFFLE_PIN, DEBOUNCE_MS);
SimpleRemote simpleRemote;

void setup()
{
  pinMode(PLAYPAUSE_PIN, INPUT);
  pinMode(NEXT_PIN, INPUT);
  pinMode(PREV_PIN, INPUT);

  // enable pull-up
  digitalWrite(PLAYPAUSE_PIN, HIGH);
  digitalWrite(NEXT_PIN, HIGH);
  digitalWrite(PREV_PIN, HIGH);
    digitalWrite(TSHUFFLE_PIN, HIGH);

  simpleRemote.setup();
}

void loop()
{
  simpleRemote.loop();

  if (play.update())
  {
    if (play.read() == LOW)
    {
      // prod the iPod in case it went to sleep since we last talked to it
      // (after which older iPods will stop responding to us otherwise)
      simpleRemote.sendiPodOn();
      delay(50);
      simpleRemote.sendButtonReleased();

      simpleRemote.sendPlay();
    }
    else
    {
      simpleRemote.sendButtonReleased();
    }
  }
  if (next.update())
  {
    if (next.read() == LOW)
    {
      // prod the iPod in case it went to sleep since we last talked to it
      // (after which older iPods will stop responding to us otherwise)
      simpleRemote.sendiPodOn();
      delay(50);
      simpleRemote.sendButtonReleased();

      simpleRemote.sendSkipForward();
    }
    else
    {
      simpleRemote.sendButtonReleased();
    }
  }
  if (prev.update())
  {
    if (prev.read() == LOW)
    {
      // prod the iPod in case it went to sleep since we last talked to it
      // (after which older iPods will stop responding to us otherwise)
      simpleRemote.sendiPodOn();
      delay(50);
      simpleRemote.sendButtonReleased();

      simpleRemote.sendSkipBackward();
    }
    else
    {
      simpleRemote.sendButtonReleased();
    }
  }
  if (tshuf.update())
  {
    if (tshuf.read() == LOW)
    {
      // prod the iPod in case it went to sleep since we last talked to it
      // (after which older iPods will stop responding to us otherwise)
      simpleRemote.sendiPodOn();
      delay(50);
      simpleRemote.sendButtonReleased();

      simpleRemote.sendToggleShuffle();
    }
    else
    {
      simpleRemote.sendButtonReleased();
    }
  }
}

by pressing buttons 2, 3, 4, 5we should be able to control a playlist of our iPhone / iPod / iPad.