I really fanatically hope this doesn't happen and the opposite happens i.e. a fuller version of iOS is used on iPhone/iPad. The revolutionary step would be to open up iOS more while still maintaining the same level of security. That would be revolutionary.
You know, this makes a lot of sense, and it's in line with what Jobs would have done: "If I were running Apple, I would milk the Macintosh for all it's worth -- and get busy on the next great thing."