As more and more Android phones are coming out with high resolution displays, along the same line as Apple's iPhone's Retina Display, many tech journalists are comparing their screens to the iPhone. They're saying thingts like "it's lower than the iPhone, but still above the 'magic' 300 pixels per inch mark".
It's important to note, though, that the 'magic' 300 really isn't that magic at all. In Apple's announcement, although it wasn't on the slide, Steve mentioned that 300 ppi is the human eye's limit to resolve an image at a distance of around ____ inches.
Many people seem to have forgotten this last part, and are taking the number to just be 300 ppi. If we take the definition of a retina display to be a display that the human eye is incapable of resolving at its intended use distance, then for a larger display, such as a tablet or a computer, the required pixels per inch will be lower. In the case of these larger Android tablets, such as the Galaxy Nexus, which is 4.65 inches, compared to the iPhones 3.5 inches, it's probably safe to say that you'll be holding it a little bit further from your face, and thus the required pixel density will be a little lower. Thus, the difference of 14 pixels per inch makes even less of a difference than it would if they were held at the same distance.
Information courtesy of AnandTech and this DPI calculator
||960 x 640
||1024 x 768
|Double resolution iPad
||2048 x 1536
||1280 x 720
Now, let me consider the rumours of a retina display iPad. If the same pixel density were used for the iPad as the iPhone, 330 ppi, at the same size, the iPad's screen would be a massive 2558 x 1919 However, because the iPad would be held further away, it's probably safe to simply double the iPad's resolution to 2048 x 1536, and call that a retina display. That gives a pixel density of 264, according to this DPI calculator. This not only makes it much easier to achieve (a significantly lower resolution display, which is likely cheaper to manufacture, and would draw less power), but also makes it easier for developers. A simple doubling in size would provide them with the same assistance as they got when the iPhone 4 was released with its retina display.