[qwzybug]

All very good points, and I think we agree on the major bits. Reading over my last post I think I might have been unclear on the main takeaway: desktop apps are fundamentally different from mobile apps.

If Apple lets us write apps that run on both platforms, I would expect the desktop versions to include substantially different resources and have fairly divergent codebases: different nibs, different build processes (perhaps with the disparate binaries packaged into one .app bundle), different interaction metaphors etc.

UIKit would be a kludge with multitouch trackpads, and downright useless without one all together.

This is largely true, but there's more to UIKit than touchesBegan:withEvent:. In particular, UIKit's view controllers/navigation controllers/table views could work quite well on OS X (and looking at iLife '09 and the preview of full-screen apps in Lion, this seems quite likely). Further, more iPhone apps use these classes than do advanced multitouch.

Again, this is not to say that writing one app that could conceivably run on both platforms will be at all easy; I still think the OP is way off the mark about that. But it's not unthinkable.