A little late to the party, but I recently worked with John Toenjes of The Laboratory for Audience Interactive Technologies using their Mosho app to send audio, video, trigger vibrate, flash screen, etc. to audience cell phones, pretty similar to @eight 's app.  They were super helpful, their documentation was straight forward, and cues were easy to tweak.  While it wasn't quite as successful as I would have liked, it had more to do with not fully explaining to the audience how to interact with the app, rather than the app itself.  Overall, it was a great first experiment...