IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
There was a great write-up recently on displaymate that explained how the dpi on one Apple Retina display is not necessarily the same as another Retina display. To oversimplify, the pixel density needed for individual pixels to be indiscernible decreases as the distance from the viewer increases. An example would be a jumbotron type LED display, where that actual size of pixels is huge, but from a distance it looks almost as clean as an image on your household television. Thus, the pixel density on the iPad is lower than the pixel density on the iPhone, because one would in theory hold the iPad further away from one's eyes, yet both qualify for Apple's "Retina" moniker.
JimLynch has already provided an answer to this; I'd like to expand a bit on this.
In terms of pixel density, once you've reached "retina" density at the normal viewing distance for that display, there's nothing more to be gained by cramming pixels closer together so, for example, the iPhone 4 at 324 pixels-per-inch is as dense a display as you could ever need or use.
People sometimes get confused when discussing this because they think about printers and there, the printer manufacturers claim resolutions as high as 2400 PPI. But it's important to remember that printer pixels are basically bi-tonal: at each pixel there either is or is not a bit of toner or an ink droplet. To represent shades of grey (or cyan, magenta, or yellow), they NEED many pixel locations to allow "halftoning". By comparison, modern computer screens allow 256 or more "shades of grey" (really, Red, Green, and Blue) at each pixel site; no "halftoning" is required.
Okay, so we've reached the end of the road on pixel density. But there are still many places for displays to make further progress; here are a few:
o Better contrast (deeper blacks)
o Less glare
o Ability to operate (be successfully viewed) in sunlight either through brighter displays or a return to "transflectiveness"
o Faster response
o More-accurate, uniform color rendition
o For certain display technologies (such as OLEDs), longer service life
o Foldability for compact storage!
o And the ever-popular Lower Cost!