This converts the AC output from a Shimano 3D71 Dynamo hub into DC for 3 power Leds (~3.2V Vf each).  The external control of the Buckpuck is used to run them at lower power at low speeds, which avoids flickering, and also runs the hub more efficiently by not drawing too much current.  The BuckPuck efficiently converts the high-voltage-low-current into power suitable for the LED.  (Hub drag is roughly proportional to current, not voltage, so this reduces drag ever so slightly.)

At higher speeds more power is directed to the LEDs to avoid overvoltage (32V) into the BuckPuck, and at very high speeds excess power is dumped into a power transistor and whatever external heat sink suits you.

This design is a little large, in terms of PCB real estate, but it is intended to be vibration-proof and includes holes for zipties to secure the large capacitors.  It can also be adapted to different numbers of LEDs by swapping out various resistors.

This particular PCB has not been fabbed and tested yet, but the circuit (based on an earlier PCB with one mistake) has been tested, and with corrections, works as intended.

More info here: http://dr2chase.wordpress.com/2009/04/25/lighting-control-circuit-fixed-working/