You're on the right track with ppi - but it's size and distance dependent.
If you hold an Evo a little further away than the iP4, so that they're apparently the same screen size, then you may find that there's no apparent difference in that in each case, the eye cannot distinguish pixels - only integrated lines.
Resolution is important, but it's over-marketed and surrounded by hype.
Take a 32" 1080p HDTV - now put it at the end of a football field's distance from you. If you can only make out that it's on, it's effectively 1 pixel. If you can make out a detail or see more than a few colors at once - it's maybe 4 to 12 pixels, effectively - even though a 1080p TV is about a 2 megapixel display. At the right long distance, that HDTV becomes nothing more distinguishable than a flashlight.
Low res sux - but after a certain point with high-res, it just doesn't matter how many pixels you have, without also considering screen size and distance from your eyes under normal use.
At about 10", anything over WVGA is approaching overkill.
That said, someone will eventually give us 1000 pixels per inch and provide the marketing material to insist we _need_ it.