can i call that command from the arduino?
I'm not sure how this relates to having the iPhone fire the Nerf gun.
I've got a RedPark serial cable that connects to the iPhone and the Arduino, that allows sending serial data back and forth between the iWhatever and the Arduino.
Unfortunately, I don't have a Mac. My son does, but he moved into his own place and took his Mac with him, so I can't build iPhone apps any more.
An ethernet shield on the Arduino, with the Arduino as server, would let you control the Arduino from the iPhone/iPad/iEtc. (or any device with a web browser).