The front glass panel of the iPhone is reported to be constructed of Corning Gorilla Glass, a chemically strengthened alkali-aluminosilicate thin sheet glass that is reported to be 20 times stiffer and 30 times harder than plastic.

Gorilla Glass holds many advantages as the iPhone 4's front panel, including its high resistance to wear and increased strength from an ion-exchange chemical strengthening process.

LED Backlight for the LCD.

Unfortunately, the LCD panel is very securely glued to the glass and digitizer, with the entire assembly measuring 3.05mm thick. It appears that if you break the glass, you'll have to replace the iPhone 4's glass, digitizer, and LCD as a single assembly.