Regarding the speculation about a retina display:
Scaling the interface would make sense. But, how would that handle web content? Apple can't exactly design a scaled up version of every website on the internet.
The reason it works on the iPhone is because websites still take up more pixels than the 4 and 4S have to offer, even with their Retina displays. But, how would a doubled in every dimension Retina display on a computer handle that? I'm guessing most websites are designed to fit a 1280x800 monitor comfortably. On this theoretical retina display, that would take up a fraction of the screen. Either every website would have to be scaled up, and would consequently look pixelated and absolutely hideous (yeah, fonts and CSS elements, and all that would scale fine, but images definitely wouldn't)... or they'd be left at their original size and be completely illegible.