The patented Revolights bike lighting system consists of two narrow rings of LEDs that mount directly to each wheel using a series of clips and ring spacers. Lithium-ion batteries, bracket-mounted to the front and rear hub, supply power to the LEDs. The batteries are slim and lightweight, and can be charged via USB. A small, fork-mounted magnet and an integrated accelerometer provide speed and orientation data to the rings. This allows the LEDs to illuminate only when oriented at the front or rear of the bicycle

http://revolights.com/pages/city

Quite neat, bit pricey.