This is a programming question! Read on before you vote to close!

According to Apple, the iPhone 4 has a new and better screen resolution:

3.5-inch (diagonal) widescreen Multi-Touch display
960-by-640-pixel resolution at 326 ppi

This little detail affects our apps in a heavy way. Most of the demo apps on the net have one thing in common: They position views in the believe that the screen has a fixed size of 320 x 480 pixels. So what most -if not all- developers do is: They designed everything in such a way, that a touchable area is -for example- 50 x 50 pixels big. Just enough to tap it. Things have been positioned relative to the upper left, to reach a specific position on screen - let's say the center, or somewhere at the bottom.

Edit: It seems Apple has integrated an switch that allows to tell if an app is highRes or not. Nice.

When we develop high-resolution apps, they probably won't work on older devices. And if they do, they would suffer a lot from 4-times the size of any image, having to scale them down in memory.