Retro emulation on smartphones has long been exclusive to Android devices, due to Apple's somewhat stricter policies relating to what apps can and can't be distributed on its App Store.
However, that's about to change; Apple has just announced that retro emulators will be permitted on iPhone – although such apps must still comply with “all applicable laws,” so don't expect any such emulators to come with pre-loaded games.
This move appears to be intended to counter the fact that iPhone users in the European Union can now use third-party app stores on their devices thanks to a recent legal ruling. Apple, it seems, wants to give its customers the chance to sample a wider range of apps on the official App Store and therefore avoid the issue of them looking elsewhere. We should note that Apple's rule change isn't just limited to Europe, though; it's a global deal.
It remains to be seen which emulators will make the leap over to iPhone, but we'd imagine any developer who already has a successful emulator on Android will port them over to Apple's phone.
Previously, iPhone users who wished to emulate retro games on their devices would need to jailbreak them, a process which Apple takes a dim view of.