Having said that, if the success of Apple has taught us anything, there is and probably always will be a sweet-spot where the technology and liberal arts… sorry, humanity – or most of it – meet. When the first iPhone came along, it did push what the technology of the time could deliver. For example, the battery life was abysmal compared to the then-standard. But it did not push too much compared to what it could deliver in return. Had the iPhone had a battery life of five minutes, it would have been useless. But it had enough. Same with price, it was expensive but not too expensive compared to what it could deliver. Had it cost one million dollars a piece, it would not have been such a success. If the size of the screen (not that the resolution was anything big there) would have meant beautiful static images but crappy performance, it would have lost the charm completely. And so, just growing resolutions on a mobile device for the sake of it would not be wise. You have to make it count as a package.

Where does that sweet-spot lie, then, for a smartphone? I’d say for a 3.5″ device Apple’s iPhone 4/4S Retina (960 x 640, 326 ppi) has the resolution pretty much nailed (otherwise those screens don’t compare all too well to AMOLEDs). Others of course made relevant ppi-to-eye-capability observations above, there is a limit beyond which the returns are diminshed. 300 ppi is quite common in printing too, I take, so perhaps there is some larger truth there. 300 ppi. More importantly, current mobile technology is not very well suited to running content at levels above that. As you take that sweet-spot to a larger frame, larger display, you obviously approach the 720p territory (1280×720), at 4.8″ the Galaxy S III has 306 ppi. Keeping things at the 300 ppi level and letting the rest of the technology catch up might be wise, and then perhaps push towards 400 ppi to loose the final traces of pixels for most people – maybe not beyond that. After that the improved performance could be directed towards improved content as opposed to improved pixel resolution, where it would make more sense.

Finally, the watching distance matters too, as observed in the home theater industry, so this depends on how the display is used.
If it is placed right on the eye, more resolution is needed to reach the same natural-like image reproduction, than if the screen is a few meters away. So, the a lot will also depend on how mobile technology evolves.. Are we going to keep our displays an arms length away or somewhere else… Yet, even though many people do not see the difference between 720p and 1080p, the film and home theater industries are already moving towards 4K. And when 4K finally comes to that sweet-spot of usability and accessibility (including affordability) for your TV or projector, you will want it. You know you do. And when 4K comes to your phone, you will want it. You know you do. 1000 ppi.

And who knows, you might want to look at your images under a microscope or something. Reality does not stop at 1000 ppi.

saying that… you do have a phone right up to your face! whereas you sit some distance from your TV! So maybe 720p would be as far as it needs to go!