Using the iPhone to control a remote controlled car via Arduino
The biggest problem you've got is how to get data from the iPhone to the Arduino.  The main possibilities are to:


Physically interface the iPod Touch/iPhone to the Arduino.  (See this thread).
Send a signal from the iPod Touch/iPhone via WiFi (or, I believe, from the iPhone via Bluetooth) to a computer (likely running a webserver) or directly to the Arduino (provided it has additional hardware to receive bluetooth or WiFi signals.)  (See this blog entry).