Apple might not be letting game developers make native iPhone games yet, but there's plenty of coders out there getting hands-on with the handset anyway. Unsurprisingly, retro game emulators are a popular choice.
Take gpSPhone, for example. It's an emulator for the Game Boy Advance, which has just been released in version 0.0.5 (i.e. very early, so don't get your hopes up).