The iPhone 4's high-density "Retina Display" is truly amazing. It looks like a glossy high-quality print on the screen rather than a regular LCD.
I know that it's probably easier to invent something like this for a smaller device, but what really needs this kind of display is the iPad. Compared to the iPhone 4 (or the high-resolution MacBook Pros for that matter), the iPad's screen is terrible for reading.
Let's hope for a quick iPad update.