gistfile1.py

#!/usr/bin/env python

import sys, os, subprocess

launch = True
blocker = ""

apps = ["Spotify", "Songbird", "VLC"]

ps = subprocess.Popen("/bin/ps -x", shell=True, stdout=subprocess.PIPE)

for line in ps.stdout.read().split("\n"):
    for app in apps:
        if app in line:
            launch = False
            blocker = app

ps.stdout.close()

if launch :
    os.spawnvp(os.P_WAIT, '/Applications/iTunes.app/Contents/MacOS/iTunesX', sys.argv)
else :
    print "Not launching iTunes while %s is running." % blocker