Yep, if Microsoft's Surface Pro paradigm ever seriously threatened the iPad, Cupertino would be out with a hybrid OS at the next annual developer conference.
One OS to rule them all, Windows 8.x, is a strategy MS have copped flack for. But if there's one company that could pull it off, it's Apple with their design sensibilities as viewed through the reality distortion field.
Apple have already reskinned parts of OS X for touch - e.g. iWork and Safari, so there's enough commonality to port useful programs between platforms. The app stores are in place, what's needed is a unified API, a common subset of OS X and iOS, that deprecates any 'legacy' NextSTEP code from the 80s. They've migrated developers before with Classic and Carbon.