I use my iPod Touch with the iForce app quite a bit now, since I always have my iPod Touch on me. The two main effects that I do:
1) An effect that comes with the app, where you make a prediction in the Doodle app and then exit out of the program and place it face-up on the table. You have your spectator (volunteer, at this point) trace their finger around in a circle around the iPod Touch, and stop whenever they feel like it (usually, to make it somewhat believable (i.e., no magic powers), I will place my hand on their shoulder (after having asked first), and mention that I will be using multiple forms of suggestion (physical and psychological) to get them to stop where I want--it will seem like they have a free choice, but really my "suggestions" will lead them where I want). And, of course, no matter where they stop, we'll open up the app again, and I'll have correctly predicted it.
2) A simple prediction effect. This one I shape around psychology (having majored in Psychology, it seems like a logical explanation), and only perform after talking to them for a bit (or, if I must, by asking them a few questions first). I'll talk about how with what little information I know about them, and based on my observations of how they act in a social situation, I can tell enough about them to make a little bit of a prediction. So I'll have them think of ANY colour, as I write down my prediction on the Doodle app, and then place it on the table face down (or exit the program, so I can leave it face up). When they reveal what colour they were thinking of, it'll match my prediction perfectly.
Very simple to do, but very powerful effects.