Don's post is spot on, that it would be
possible to run retina apps 2x on a pixel-per-pixel basis but it's not optimal and furthermore, not
sensible. Not for most users, and by consequence not for Apple. Does anybody honestly
like using 2x apps on their iPad?
If the answer is no, then would the reason be because they appear pixelated, or because the UI looks and feels sort of
cheesy compared to using a
real iPad app?
Smoothing out all of those "jaggies" wouldn't "magically" make the "2x app experience" acceptable, now would it? The 7" Galaxy Tab and the soon to be running 2.3 Android apps Playbook comes to mind.
Let's face it, the "blown-up phone app experience" is
bogus and no place any one of us really wants to be, and so Apple made some
philosophical rules with their product to nudge developers to do the right thing and either choose one of two screen sizes for the application, or make it a universal binary.
At the end of the day, this "rule" and the path it will forge
moving forward is the best thing for everybody, although there may be some casualties along the way because the philosophy at Apple is that the user experience wins out at the end of the day.
Mo' magic, mo' money.
Well, the short answer is that it would be "possible" but not optimal.
The UI Elements on iPad and non-retina iPhone/iPod touch are scaled work with a finger "touch area" of about 40 x 40 pixels.
The UI Elements on a Retina display iPhone/iPod Touch 4 are scaled to work with a touch area of 80x80 pixels.
If you just put the retina display App (960x640) on the iPad screen as-is, the various UI elements would be too big and look awkward. The UI's really need to be retooled to look right on iPad. As some Apps draw a lot of their own UI, Apple couldn't just "automatically" rescale the UI elements.
This is basically the same weird look you get with a lot of iPhone Apps that run at 2x on iPad.