I'm a contractor and have seen table saws that can detect contact with a human finger / hand and cut the motor power instantly. It uses a current sensor similar to what you see in your kitchen and bathroom outlets aka GFCI outlets. That wouldn't work on a copter as the props aren't conductive and it most certainly isn't grounded but I wonder if a similar idea for safety could be done with our copters that would detect propeller impact if the controller could detect a sudden rise in current draw from one or several motors that wasn't correlated to stick or controller commands? If a powered motor is physically slowed down or suddenly stopped doesn't it draw more current or would there be some way to detect this?

Perhaps another idea could be propeller rpm detection. If a light weight rpm sensor could be on each motor arm  and if the controller saw a sudden stoppage or significant slow down it could kill all power to all motors for safety. I have a battery operated one just for my glow engines. Way too big but perhaps they could be made much smaller. Who knows how this data could also be used to help with the flight control itself?

Neither of these would prevent the initial contact but anybody who's even seen a copter mishap knows it's the few seconds that often follows where it's still going full bore that can really do the damage.