We are looking to use the Ambient Light Sensor on iOS. It seems that there is a public API in the IOKit framework, however it has a disclaimer at the top that says "Although it is a public framework, Apple discourages developers from using it, and any apps using it will be rejected from App Store."
Looking at other forum posts, most people want to use the Light Sensor in order to adjust the brightness. Adjusting brightness will definitely get your App rejected by the App Store. What we are wondering is, will Apple reject any App that uses the light sensor, or only those Apps that use the light sensor to adjust the brightness?