I haven't noticed it with the DINC2 yet, but my D2G had the same thing. Never noticed the correlation, but it makes sense. The light is probably used with the prox sensor.
EDIT: Used Elixir just now, and best I can figure, it's used to as part of the prox sensor. I did see it when I turned the sensor on, so the sensor is likely only activated normally during a phone call.
Current: HTC Droid Incredible 2 and Motorola Droid 2 Global.
Former: Blackberry Bold 9700, iPhone 3G, HTC Fuze, Samsung Epix, Treo 750, Cingular 8525, Cingular 8125, Audiovox SMT5600, other non-important junk.