That solution is a bit hacky and may not work in future versions.
Do you mean the F key play button?
EDIT: The Play button doesn't launch iTunes when I have QuickTime Player already open. Perhaps because they both compete for the button.
If you want an different application to launch, then you want to use the F-key function, instead of the Apple keyboard functions, and assign that to the app.
There's a free system preference called FunctionFlip that allows you to control individually which function keys use the Apple keyboard functions and which use the standard F-key.
To assign an app to the F-key, create a Service that launches the app, and then assign it to the F-key in System Prefs > Keyboard Shortcuts.