A patent that Apple filed back in May shows how your iPhone could act as a disco light and sync to your music. The iPhone disco light sounds like it would work with a special processor. Another possibility is having the iPhone act as a safety strobe light for runners who exercise in the evening (via 9To5Mac).
The first part of the patent that details the iPhone disco light says:
A processor configured to control a light level transmitted by display according to a playlist, which is selected from the plurality of playlists, wherein the processor is configured to control the light level using a preset intensity pattern assigned to the selected playlist […] The system of claim 4, wherein the music mode varies the light level transmitted by the display in accordance with the song being played.
The second part discusses how the iPhone could repeatedly flash as a warning to cars:
The computing device has a light source configured to output a beacon light effect that alerts others to the presence of the user of the computing device when the user is in low light or no light conditions. The beacon light effect may include blinking, fading in and out, ebbing, pulsating or strobing.
This is a continuation of patents that date back to 2004, as suggested by an illustration of an iPod Classic in the patent filing.