Doesn't just pressing play/pause (F8 on this Apple aluminium keyboard) launch iTunes?
More generically, I happen to be using BetterTouchTool to improve my use of the Magic Mouse and my touchpad, but it also includes keyboard settings to launch apps from keycombos, and many other things. You could try that. IT's under Gestures, Keyboard, add new shortcut, then pick a keycombo and choose Open Application from the predefined action popout.