ooh.. said:
Bigger screens don't need a high ppi because you'll be looking at them from further away, like a 1080p 40in tv, the ppi on one of those will be much lower than most smartphones but you'll be sat at least 4 or 5 feet from it hence it'll look sharp, but have a look at one from about a foot away and you'll see the pixels.
Software can't overcome the problems a low ppi brings, the HTC Sensation XL has a fantastic screen, very bright and punchy with good blacks and great viewing angles, but at 4.7in and with a resolution if 800 x 480 text appears jagged when zoomed out, the GS2 to a lesser extent as its screen is smaller, but you can still make out the pixels.
As I say, once you cram enough ppi in at a given screen size then This isn't a problem, it seems all of this years high end smartphones will have enough ppi, catching up with Apple 🙂