Here's something interesting: the "AI Motor".
The manual for the thing is here:
This is a 4-wire device; power, ground, transmit data, and receive data. Talks to a serial port. These devices are addressable, can be daisy-chained, and talk what seems to be a reasonable binary protocol, with useful commands like "Position Set", "Position Read", and "Control Gain Set". Price about CAN$77 each.
No idea if this is a good device, but at least we're finally out of PWM servo interfaces and into a real protocol, where you can find out what the servo is doing.
John Nagle