Here's how the trick works. You launch the app and the phone does some nicely rendered videos with a curtain opening and some show-bizzy music. Screen prompts ask your innocent assistant to pick one of three objects and say the name of the object out loud. Without further ado you hand the person your iPhone and with music, a puff of smoke and some suspense the phone announces the object the person picked. Yes, it can be repeated, and the effect would be pretty astounding for most people. Fun to try around the office cubicle, or at a bar, or anywhere really.
It's $2.99 at the App Store and worth it for the fun and mystification it can provide. My only suggestion is that there shouldn't be an obvious link to the performance tutorial on the application's main screen. If someone is browsing through the apps on your phone, they are only one click and a 90-second video away from the secret.
Try it and see if people are fooled. I predict they will be.