You could communicate over serial directly. You can build your own serial cable with a breakout board, like the PodBreakout. On the breakout board, Tx is 12 and Rx is 13. I can confirm serial works with iOS 7.0.4 on an iPhone 4, but it must be jailbroken to be able to do anything with serial. On the Raspberry Pi, Tx and Rx are to be pins 8 and 10, respectively, on the Model B Rev. 2.
I've only used the iPhone and serial with an Arduino Uno, but there's no reason it wouldn't work with the Raspberry Pi. The Pi's GPIO pins are 3.3v, so you should be able to directly connect the Pi's serial pins to the iPhone without any voltage issue.
Check out this article for a bit about using the iPhone and serial communication. It talks about Arduino and some other things, but the same essentials apply to the Pi. I know you can access the Pi's console over serial, but I'm not sure how you could do that with an iPhone. You might be able to do it with minicom, if you can get it set up.