And we will see whether Apple will allow such app in their store. I would imagine they will not, because they would rather have people using their own maps, but perhaps they will be benevolent this time.
If I had to guess, Google will attempt to get it into the App Store before iOS 6 is released. Apple will probably end up delaying approval of the app for a few months after iOS 6 to let users become accustomed to using their own solution.
Apple seems to have moved away from disallowing apps for "duplicate core functionality" - we have GVoice, Sparrow, Camera+ now. I don't see how they could not allow it.