The answer is Gorilla Glass, developed especially for mobile electronics.
Walter Isaacson, in his biography of Steve Jobs, tells it this way:
"For the iPhone, the original plan was for it to have a plastic screen, like the iPod. But Jobs decided it would feel much more elegant and substantive if the screens were glass. So he set about finding a glass that would be strong and resistant to scratches."
A friend told Mr. Jobs to talk to Wendell Weeks, the young and dynamic CEO of Corning Glass in upstate New York.