If you're looking for a good magic trick to show off your iOS device, give Magic Trick #1 a try (US$0.99).
When you perform the trick, the spectator sees several cards. He or she names either the pip or picture cards, the type of card and the suit. You as the performer pick up your iPhone or iPad, which shows a black screen. You then rub the screen, scraping away the black and revealing the chosen card.
The magicians out there will have some ideas of how the trick works, but I have to say there are some subtleties that flow so smoothly from your actions that I don't think most people will catch them.