300 dpi isn't that far away from reality.
At least two companies have showcased 200 dpi screens, Viewsonic and IBM. IBM was even selling one, the T221.
http://tinyurl.com/dahmq
As for current graphics "looking ugly when scaled", what are you thinking of? If you have a 300 dpi screen, and show 72 dpi graphics scaled to 400% (actually 1600 but who's counting), they will look identical to 72 dpi images presented at 100% on a 72 dpi screen!
Same thing goes, naturally, for icons and everything else. No distortion will be visible, as long as integer scaling is used.
The real problems are cost, cost and availability. Cost, because the screens are really expensive, $8000+. Cost again, because the video cards are very expensive, $1000+. Availability, since they aren't selling them anymore, probably because most people just can't justify spending more than nine thousand dollars on a screen and video card.