DPI and PPI would both equal to 300 each where the human eye cant see past, even up close.
I'm having trouble making sense of this sentence, but what you seem to be doing is equating 300 DPI in printed material with a 300 PPI screen. Sorry mate, but I think you're confusing some principles here. The 300 DPI figure which is commonly thrown about in printing, is actually referring to the typical resolution an image has to be in order for it not to appear pixelated when printed using halftone screens. The critical factor here is not the human eye, or how far away you view the printed material. The limiting factor is the the screen ruling, which is a measure of the number of lines of dots in the halftone screen. Basically, when printing an image, the resolution needs to be at least one-and-a-half times your screen ruling. So if you have a screen ruling of 150 LPI, then your image needs to be upwards of around 225 PPI. If you were using 180 LPI, the image resolution should be upwards of about 270 PPI. So 300 PPI happens to safely cover most printing needs, and it's a nice round number that's easy to remember. There's nothing to stop you using higher resolution images, but you won't see any difference in the quality of printed output, and again, it has nothing to do with how close you view the printed output. (Incidentally, when you print a solid colour rather than a halftone, like black type in a book for example, the actual resolution is much higher than 300 DPI! This is where the viewing distance would make a difference, but pick up a good quality book and hold it as close to your eye as you can focus, and you won't see jaggies in the type. It will appear smooth, and that is because it's been printed much, much higher than 300 DPI.)
The human eye has trouble seeing individual dots/pixels past 240dpi/ppi, but everyone is different so not everyone has the trouble.
Regardless, its a marketing term that should be a tech spec that deserves the term retina display which unfortunately it doesnt because its too loose of a term.
Sure, it's a marketing term! But it describes an aspect of the product that is absolutely relevant to the user. It's never been the goal of a display to reproduce graphics with visible pixels, like a Lego-block creation. That's just always been an inherent weakness of the technology. The ability to create the appearance of smooth, flawless lines and curves is the dream! And by all reports, this iPad screen, like the iPhone 4 screen, does that! How cool is that? I think reaching a milestone like that deserve a memorable marketing name if anything does! Now of course the ability of people's eyes to detect pixels is going to vary somewhat, and different people are going to hold the device at different distances from their faces and that distance is going to vary depending on how they're sitting and holding the device
So yes, of course, we're working with a slightly loose set of numbers here
but that's the nature of what the term describes! At the end of the day, if most users won't be able to discern individual pixels in normal use, then I think Apple has every right to market this facet of the product.
But hey, if you think a number like 240, or 264, or 300 is a more relevant descriptor, go look at the tech specs. They provide these numbers too.
