Hooking up a Redpark Cable
I recently started a project that involved building a simple circuit controlled by an Arduino supervised by an app running on an iPhone/iPad. The simplest way to get an Arduino hooked up to an iPhone or iPad is the Redpark TTL Cable which you can pickup at Maker Shed for about $60.
There's a great ebook you can grab called iOS Sensor Apps with Arduino, though in the book it talks about an alternate Redpark iOS-to-RS232 cable that requires an intermediate adapter to hookup to an Arduino. You really want the iOS-to-TTL version I linked above since it can be connected directly.
Here's how. The white jumper is ground on the cable to ground on the Arduino (I know, tragic color choice). The blue jumper is going from the blue wire on the cable (TX) to RX on the Arduino and the red jumper is going from purple on cable (RX) to TX on the Arduino cable.
There are a couple gotchas, though. First, your cable will interfere with the USB cable and communication with the computer. Get in the habit of only having one connected at a time. This means you'll need to power the Arduino with the power adapter or a battery pack.
The other gotcha has to do with unplugging the Redpark cable from the phone. This will often cause the phone not to recognize the cable when it's plugged back in until the Arduino board is reset. I suspect a more robust connection that utilizes the RTS/DTR/CTS pins might resolve this little annoyance.