300 PPI is the normal definition for "print quality" at a distance of one foot. (Apple marketing used the same calculation method, but changed the name to "retina" so it sounded cooler and could be used in multiple situations.)
The closer you get, the denser it must be to stay "retina".
At 1" away, many people can theoretically see over 3500 PPI.
Depends on the "regular viewing distance" and whose eyes. (Many people have better than normal vision, many people do not.)
Here are some distances for various PPI to qualify as "retina":
Code:
PPI Dist Device
=== === =========
595 - 6"
357 - 10"
326 - 11" - iPhone 4 ('Retina')
162 - 22" - iPhone 3GS
132 - 27" - iPad
That's for normal vision. Someone with excellent vision might have to hold a "Retina" iPhone much further away.... up to 18" instead of just 11".
It can make a visual difference. Many modern printers go way past 300 DPI printer/retina quality, for example.