I am a piezoelectric transducer design engineer, so I am pretty sure that these are based on the piezoelectric polymer: PVDF (polyvinylidene flouride). They act similar to the standrd ceramic under-saddle pickups, and they really aren't anything dramatically new. The advantages are that they are very thin and flexible. This disadvantages are that the have low output and low capacitance (high self-noise). Also, for under-saddle pickups, the stiffness of ceramics is actually an advantage. I doubt that these will be applied to real microphones, but will probably always be an option for instrument pickups, where they can be physically attached to a vibrating member.
Peace,
Rick