Steve Jobs in the key note himself said in Apples' keynote, that anything with over 300 pixels per inch is "too detailed" that each individual pixel is invisible to the human eye.

So going by Apple's definition, a "Retina Display" is any display with over 300ppi.

The iPhone 4 and 4S easily achieve this (329ppi) because they have such small (3.5") screens. The Nokia E6 does have a "Retina Display" (326ppi) because the 640x480 pixels are squished into a 2.46" profile. Disappointingly The Apple iPad (3rd gen) does NOT have "Apple Retina Display" since its only 264 ppi, none of the previous versions do.

Hell even my SG NOTE doesn't have it, at only 285ppi. The Gnex has a smaller resolution than the NOTE but it does have a Retina Display (315ppi) thanks to its smaller 4.65inch size, however it feels inferior to the NOTE from my own user experience. So to be honest, anything over 250ppi is really going overboard. I rather companies put that extra effort into making the screen with deeper blacks and brighter whites, or other characteristics such as screen reflection, finger smudges and viewing angles.

I mean an iPhone with 960x640 resolution on a 4.6inch (251ppi) space would be ideal to the user to make better use of that space to add in more buttons/icons and better viewing experience (movies, browsing etc). And it would only be marginally larger than the current version with an edge-to-edge display (little bezel).

Samsung is realizing this, while Apple is regretting it.
iOS is dependant on pixel count, whereas Android is scalable...making Android more future-proof and always one step ahead.