this graphic is idiotic (I hope someone already noticed this an commented btw).
What the heck is the graphic supposed to show and why are you reposting it?
HDPi displays are about high dpi, smaller pixels per set area in simple words, it's not about setting an x mm pixel size.
This way visually one who wouldn't know better (to no fault of their own of course) comes to the moronic conclusion that a blue ray screen and an ipad 1 screen are better than an iphone 4, 4s. They are not, they are worse, since their "grain" size is higher than the iphone 4,4s. Of course at a distance of a blue ray screen this difference with the iphone 4 or 4s hardly makes any difference, but from ipad 1's distance of usage to that of iphone 4's it makes a whole lot difference, and the ipad 1 screen is worse. So what does this graph represent? Nothing.
The comparison graphic would be to choose
a set area then compare different pixel count on each one. Because it's the number AND size of pixels per area, and obviously higher no of pixels in the same area means smaller pixels which is the intention.
Anyway, I am very much looking forward to a retina display for the ipad seeing as my eye-sight is in dire need of one. Having said this two other points are also very important in screen quality: that they fuse the glass with the screen some way a la iphone 4,s so we don't have two surfaces of potential glare, and secondly that apple finally get's involved in good antireflective tech in their products by investing serious $$ on it and not merely sticking almost untreated glare inducing glass on everything.
(for more see here:
http://www.pcmonitors.org/articles/matte-vs-glossy-monitors )