As well as the problems mentioned in other comments, this idea suffers from a massive "analog hole" just as with audio. If you can print an object, it would be extremely easy to sample the pulses sent to the stepper motors and run a "virtual 3D printer" algorithm on those waveforms to turn it back into an unencrypted 3D model suitable for unlimited printing on any printer. The only way around this would be to forbid 3D printers from printing unsigned objects, and forbid people from making their own (unencumbered) 3D printers. Given how many people have made their own repraps, that's already a non-starter.