Today's New York Times has a story on the controversy (here), as well as this graphic of how such a motor can be hidden with the bike tubing to be virtually invisible:
However, two seemingly unrelated points made in the story have convinced me that this form of doping is highly unlikely to have been used by the pros. First, the story notes that because of battery-life limitations, the motor is likely to be used only for short bursts of extra power over the course of a long race. Second, the motor adds approximately 4 pounds to the weight of the bicycle. That is a hell of a lot of extra weight for any rider, let alone a pro, to drag around on a bicycle. It strikes me as highly unlikely that any rider in the pro peloton would willingly add 4 pounds of extra weight to his bike to obtain the advantage of short bursts of extra power, especially in a mountain stage. The extra weight would likely more than counteract the short-term power advantages offered by the motor. What purpose would it serve to activate the motor when the rider is already a mile or more behind the peloton because he has been dragging around an extra 4 pounds for the past 75 miles?
Moreover, the excess weight should make motors easy to detect by race referees, who often weigh bikes to ensure that they are within UCI weight limitations (i.e., that they do not weigh too little). After weighing a bunch of bikes that come in between 15 and 16 pounds, a bike weighing around 20 pounds surely would stand out as much as a rider with a beer belly.
We might predict that improvements in battery technology will lead to lighter motors in the future. When those innovations occur, there surely will be a threat of motorize riders in the pro peloton. But it seems far fetched to suppose that any pro would use the current, heavy technology (unless they can find a way to shave pounds off their bikes in other ways).