There's the Redpark TTL Cable which allows serial communication of iPad, iPod, iPhone, etc. with your Arduino very easily (and officially, without jailbreaking):
Or you could use a Bluetooth Low Energy shield:
Either this one:
http://www.seeedstudio.com/depot/bluetooth-low-energy-shield-p-1255.html?cPath=132_134
Or this one:
http://www.kickstarter.com/projects/rowdyrobot/arduino-ble-shield-connecting-the-ios-and-the-ardu