Check out Fire Fly.
The Firefly light uses a Passive Infrared sensor in order to detect traffic approaching from behind the rider. Upon detection LEDs flash onto the back of the rider with varying intensity depending on the proximity of the traffic. The protruding arm of the device allows the light to illuminate the entire back of the rider at any angle. A set of LEDs also illuminate the ground beneath the rider; providing traffic with a proximity of depth which can be used to determine exactly where the rider is. While a rear tail-light provides strong rear presence and meets bicycle light requirements.
Not sure if it's in production yet, but still a really great idea.