I don't understand the obsession people have with Nintendo releasing their games inside Apple's walled garden.
Nintendo already releases really great games, on good hardware specifically designed for playing those games. It's widely available, designed well for it's intended purpose and priced very competitively (32gb iPod Touch is $250, 32gb Wii U is $300 but also usually includes a ~$50 game).
They also sell a reasonable back-catalog of their older games. Their Wii U hardware is backwards compatible with nearly everything they've ever released (hardware-native compatible with Wii and GameCube, software-emulates N64, SNES, NES, GBA, DS, etc).
Nintendo isn't perfect by any means. But their setup right now is pretty competitive, and their hardware has been intelligently selected and priced.
Nintendo might gain a little bit of sales by jumping into iOS, but they'd loose a lot too -- way more than most people might realize.