Pixel Density of the iPhone 4

By developing pixels a mere 78 micrometers wide, Apple engineers were able to pack four times the number of pixels into the same 3.5-inch (diagonal) screen found on earlier iPhone models. The resulting pixel density of iPhone 4 — 326 pixels per inch — makes text and graphics look smooth and continuous at any size. (via Apple - iPhone 4 - Learn about the high-resolution Retina display)