In a patent titled Secondary backlight indicator for portable media devices, Apple describes a dual backlighting system where a second backlight below the primary backlight of a display could show visual notifications of various events. The idea is to offer a low-power, flexible notification mechanism that can work even when the iPhone's primary display is off.
"For example, each indicator may have a particular color, blinking speed, or light intensity. These properties may be programmed into the electronic device by the user, or may be hard-coded or hard-wired into the system," Apple said. "Thus, when the secondary backlight is turned on, the properties of the light provided by the secondary backlight system may depend on the status of the device. Also, if the status of the electronic device changes while the secondary backlight is on, the properties of the light provided by the secondary backlight system may change to reflect the new status."