Originally Posted by steelbikeguy

the general touch switch relies on the user's body picking up the nearby AC waveform from adjacent mains power. I'm not sure that this would be sufficient on a bicycle used outdoors.

There could be ways to use a variable capacitance, though. You could set up an oscillator that would change frequency as the capacitance changes. Another circuit could watch the oscillator frequency and turn the light on or off when the frequency changes.
The downside of this is that the circuit will be using power all of the time, albeit not a lot of power.

Maybe you could just use a little reed switch to control power to the light? This just requires placing a magnet nearby the reed switch to turn it on. One of the SON lights uses this idea... is the Edeluxe?
The nice thing is that if you've got an old bike speedometer laying around, it has a reed switch that you can use! Most of them use reed switches as the sensor.

Steve in Peoria