IPad – At what resolution will an iPhone only app run at on an iPad Pro

The German magazine c't (or rather the new "Mac & i") tested the new iPad (3rd gen.) and found that it finally uses retina graphics in the iPhone apps that provide them:

Übrigens stellt das iPad 3 auch Apps, die für das Retina-Display des iPhone 4(S) optimiert wurden, schärfer und ansprechender dar als sein Vorgänger – iOS zieht nun also endlich die optimierten Grafiken heran. Das wirkt sich positiv auf beide Darstellungsvarianten aus, auf die Originalgröße und den "2x"-Modus.

Translation:

By the way, apps that are optimized for the retina display of the iPhone 4 (S) are displayed by the new iPad sharper and more appealing than by its predecessor - iOS now finally uses the optimized graphics. This has a positive effect on both display variants the original size and the "2x" mode.

I haven't found a detailed list of the exact behaviors. For the most part, it seems that everything is simply pixel-scaled (which is why text looks bad), except for images which were scaled down to begin with. I presume that the UIImageView class has some behavior to handle this automatically.

On Retina devices, Apple's built-in text and image rendering are aware of the scaling factor of the screen, and will render text and photos in high quality. But this doesn't seem to apply to iPhone apps run on an iPad. (See Apple's developer documentation on high-resolution screens for more information on that.)