The problem with automatic on/off is that there is no easy way to do it. It'd either require GPS or a wheel sensor. Sure, you could probably get away with an accelerometer, but technically if you're an extremely smooth rider you could trick it into thinking you're stopped when you are actually moving. (I doubt someone could achieve this.) There is no force when you're moving at a constant velocity, so the accelerometer would output nothing in that instance.

The Velodroom is a light that does have accelerometers in it for auto on/off, but I doubt it'd be cheap and it looks like it's only currently a taillight.