Think of something like MS Outlook - it's a big, complex app with lots of features. Now imagine that you only write the app once, but design two interfaces for it - one set for the Mac, and the other for the iPad. You compile through XCode, and upload to the App store. Apple notes that their are interfaces for both iPad and Mac, and posts it in both app stores.

Obviously the mechanics would not be exactly like this (I presume a manifest of some kind), but it should be pretty close.

For Apple, this would bring more developers onto their tooling (yes, even many of those that complain about XCode), and allow more developers to target multiple platforms with little extra effort.

In fact, one of the more interesting things is that Apple could even create new platforms that just require some interface additions and an updated manifest.

Have a bug in your code? One fix. One upload. One code review.