In 2004 I was using a 2048x1280 display at 112 ppi. Today I can't find a screen over 100 ppi. So in pixel density we have actually gone backwards, except for laptops.
hi ppi IS useful. you don't need antialiasing software and such crap if you have hi ppi. personally, I would be able to find use for a wqxga (2560x1600) in a 14" or 15" laptop screen. And no, font size does not become tiny if one does not want this to be (although I want tiny fonts), with the new desktop technologies like microsoft aero or linux counterparts like plasma and compiz.
While pixel density obviously isn't making leaps and bounds, Apple's products are pretty consistently over 100 ppi (not to mention the iphone at 160 ppi):
My 2 year old 15 inch ThinkPad T61p is 1900 x 1200 which is 150ppi. And honestly I don't really want a higher ppi. Software that does not let you change the default fount size becomes almost unreadable. It all comes down to viewing distance and the further you sit from the screen the less useful high PPI becomes.
