Apple has called their iPhone 4's display "retina display", and they explained that having over 300 PPI at 12" distant from the eyes, means that you can't see the pixels, and that it's like paper. But after that some display experts have argued that it's not really true:
"Steve Jobs claimed that the iPhone 4 has a resolution higher than the Retina - that's not right:
1. The resolution of the retina is in angular measure - the accepted value is 50 Cycles Per Degree. A cycle is a line pair, which is two pixels, so the angular resolution of the eye is 0.6 arc minutes per pixel.
2. So, if you hold an iPhone at the typical 12 inches from your eyes it would need to be 477 pixels per inch to be a retina limited display. At 8 inches it would need to be 716 ppi. You have to hold it out 18 inches before the requirement falls to 318 ppi. The iPhone 4 resolution is 326 ppi.
So the iPhone 4 has significantly lower resolution than the retina. It actually needs a resolution significantly higher than the retina in order to deliver an image that appears perfect to the retina."
As you can see, he says that the "real" retina display is actually closer to 500 PPI than 300. The new iPad has only 264 PPI and yet they are still calling it "retina display". The new Macbook Pros will probably have an even lower PPI, because they'll need a 4k resolution display to get 300 PPI on a 15" screen, and it's very unlikely they will have that in the upcoming laptops.
But as mentioned above, the real retina is somewhere close to 500 PPI, which is something that LG has just announced for 5" smartphones that will now be able to have a 1080p screen, or 440 PPI, which is close enough. The display will be based on their AH-IPS technology (great color accuracy and wide viewing angles), which seems to have surpassed Samsung's Super AMOLED in most areas other than color contrast.
In the picture above we can see that at 500 PPI the display really looks like paper, and I'm actually quite excited to see these displays in smartphones in the coming year. First, I don't think they will be arriving in phones until next year, and second, I would wait a generation or two, until there are much stronger GPU's that can easily handle this resolution without completely draining the battery life of the phone, and posing a huge performance impact on the GPU.
Still, if they are going to be able to put this kind of resolution in a 5" display, it means it should be easier for them to put it on 7" tablets, and even easier on 10" ones. The easier and cheaper it gets for the industry to make such high quality displays, the better it is for all of us.