There are unofficial Pokémon apps, such as this. Nintendo and Game Freak are *finally* releasing an official Pokémon app for the AppStore and the Android Market.
The app, Pokémon Say Tap? BW, uses music and on screen Pokémon cards in a timing game. The free app is slated to be out this summer, making it the first Pokémon game to appear on non-Nintendo hardware.
Well, besides those puck-based Pokémon arcades. Still, this is a big deal. Nintendo's making its first move onto smartphones.
ポケモンがついに公式でスマフォ進出！ 『ポケモン言えTAP？』今夏配信 [Kotaku Japan]