So iPhone games have, to date, been largely disappointing. Let's forget iPhone games for a minute, then, at look at an iPhone synth that lets you create your own 8-bit soundtracks.
This is 8bitone, by Japanese devs Yudo, and it lets you arrange your own chiptune soundtracks on the fly. It's not a professional solution, of course, but hey, it's a program for your phone. And for that, it looks pretty good.
It'll be available on the App Store in June, but you can check out a preview in the clip below.