Non-retina iPads use retina assets for iPhone apps.
Ah good too know. I guess that is new in iOS 7.
My memory is fuzzy, but I think it happened around iOS 5.
Nope, it's an iOS 7 feature. Retina iPads have done this since iOS 5, since that is when the iPad 3 was introduced, but non-retina iPads used non-retina graphics until iOS 7.