Oh dear. Those videos, if not faked as claimed by Riis (and it will be very easy for someone to check the original footage), are compelling evidence indeed. Which is very disappointing as Cancellara always came across as an honourable sort, and he is a consummate rouleur, physically.
So assuming the video is not faked, the most compelling evidence is the acceleration viewed from the helicopter in Paris Roubaix. I remember seeing that at the time and thinking "Jesus, how can he do that", but it was on a grainy feed on the laptop and I couldn't re-watch it to examine closely, and it all happened very quickly of course (not that I suspected then that he was motorised).
Anyway, that acceleration, and similarly the one in Flanders, is just unnatural. No rider is strong enough to make such incredible progress over other riders already going that fast without getting out of the saddle and sprinting - he is that fast. The other riders weren't exactly dawdling, as you can see they were already moving away from the rest of their group who did seem to have let up slightly. Not only does he rocket away without getting out of the saddle, there is literally NO perceivable increase in effort from him - not even a slight extra crouch or tightened grip on the bars or adjustment of pedalling action or extra tensing of leg muscles; it is utterly effortless compared to what he is already doing AND he turns it on instantaneously AND he's able to turn round to see the reaction in the middle of doing this incredible acceleration.
That's all before we even get to the handlebar-shift-lever hi-jinks. The suggestion that these movements are just finger-stretching and stress relief before making an effort seems very unlikely to me, having raced myself and watched a lot of professional racing closely too. On straight sections of cobbles, you really want to be keeping your hands on the centre of the bars as much as possible, and relieving stress in the hands or stretching your fingers would be more easly achieved by pressing them against your leg or pressing your finger tips against the bars - not by repeated very precise movements to an exact point on the brake/shift lever. This should be another easy one to solve - by looking at past footage of him when he wasn't pulling off such stunning manoeuvres and searching for similar stress relieving actions - but I doubt if there are any similar episodes.
As FM says, the noise of the motorbikes and crowd will obscure the sound of the motor - but of course there is also the helicopter roaring overhead too, so no difficulty hiding the sound. What the motor designers/installers and Cancellara haven't got right is building a more natural-looking progressive action into the motor's activation. Having now had the opportunity to re-watch and scrutinise it, that Paris-Roubaix footage is almost comical, just as raindog says. But as pointed out, it's been rumbled now so hopefully the UCI will be vigilant enough to stamp it out entirely. If we see similar outstandingly effortless super-accelerations from Cancellara from now on, against people like an in-form Boonen (!), then we'll have to take it all back, but somehow I doubt very much that we will.
Then there's the odd denial. If one's reputation as an honourable rider and one's strength/supremacy were being traduced this way, surely most honourable people would be making vehement claims about how performimg clean and unaided is a matter of honour for them and that they swear on their children's/mother's/etc life that they are clean, and "please come and inspect my bikes/fridge/tour bus/mechanics' HQ/hotel room etc etc as it's really important to me that you see I am innocent". But they never do.......
. I think you are riding Mr T Roll 
) I believe Chris Boardman rather than you - as he clearly has the cutting edge info at his fingertips.
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