Back in 2010 and 2011 running Android OS on an iPhone as a dual-boot was the hottest thing. First there was Bootlace, followed by iPhoDroid and iDroid. Planetbeing was the first one to port Android to iOS, and you can watch him discuss his port here, followed by Sergio McFly with iPhoDroid and the team behind iDroid. In October 2011 Myriad announced Alien Dalvik, which was suppose to be some sort of Android emulator-thing for iOS. But the project didn’t really take off.
Was iOS ported to Android? No, but some Columbia University students managed to get iOS apps running on Android with their Cider compatibility layer. Cider is not an emulator or virtual machine, but it tricks apps into believing that they’re in a native environment: they adapt code on the fly to make it work with Android’s kernel and programming libraries. Even 3D benchmarks run properly.
Unfortunately, most iOS apps run at glacially slow pace. They also don’t have access to most hardware features. This is still better than previous efforts, though, and it raises hopes that platform exclusives won’t be as important in choosing a mobile device as they have been in the past.