Looking for an investment? Here's a start up that has a different take on a power meter:
Laser Spoke measures cyclists' power output
If they can make it a little more compact they may be onto something. It looks like crap the way it's shown.
This popped up almost two years on another forum I sometimes visit. The inventor actually chimed in briefly, but never answered several serious questions that people raised, including how the system would handle road dirt and other optical interference, and most critically, how it would be calibrated to a given wheel, since the measurement depends on the amount of actual twist experienced by the wheel, and the torsional stiffness of different wheels varies a lot.
Here is a link to the actual patent application, if you're of a technical mind.
Method and Apparatus for Measuring Torque Transmitted by Driven Wheel of a Cycle or the Like Vehicle - Patent application From a quick scan, it appears that calibration procedure that sounds feasible is described in the patent application. I don't know whether the procedure could be done with sufficient accuracy in the real world, but it's interesting.