I agree with Nick; something involving applescript is probably your best bet.
There's also a program called SerialMagic that essentially uses serial data to trigger keystrokes. I'm not entirely sure how this program works as I am not at my mac right now, but it seems that it basically coverts incoming serial data to keystrokes. It seems like you could either bind applescripts to Fkeys or just have the Itunes open and just have SerialMagic jam on the left or right keyboard keys as data comes in from Arduino.
Unfortunately the program is 40$ and I don't know how functional the demo is or whether it would actually work with Arduino. Some experimentation by someone actually near a Mac is in order.
More info is here:http://www.serialio.com/products/SerialMagic/index.php
FYI I'm currently trying to do something similar, only instead of sending serial into the computer and having it respond, I want to send serial data out to the Arduino board (in my case, I want to display my unread mail count via LEDs). Topic for a post of my own once I run out of research leads