iPhone 4's "Retina Display"2010-06-26
I got an iPhone 4 to replace my iPhone 3G (not 3GS). The "Retina Display" is very nice. Some apps work better as a direct result of the improved display. You get a lot more detail in Maps. Very high-density web pages (like the nytimes.com home page) are legible on the iPhone 4 but not on the 3G. Photos and Youtube probably benefit too, but I rarely use them.
But most apps aren't any more useful as a result of the better display. Phone, Calendar, Mail, and most of the rest look a bit nicer but don't provide any extra benefit as a result of the display.