Optional: 2 x XBee module (802.15.4/ZigBee)
Optional: HiJack Development Pack
With the iPhone/iPad/iPod to Arduino ZigBee/802.15.4 kit you'll be able to connect your iPhone/iPad/iPod to XBee module and communicate wirelessly with Arduino (using the XBee shield). Sending and receiving data from Arduino to iPhone/iPad/iPod and from iPhone/iPad/iPod to Arduino will be very easy, you may need the Redpark Serial Cable SDK (see downloads) for developing your own apps for iPhone/iPad/iPod. Add some sensors to Arduino and you can get data (temperature, light...) in your iPhone/iPad/iPod display, even you can add transistors/relays and control lights, motors... from your iPhone/iPad/iPod display.
Downloads: Redpark Serial Cable SDK