This Analog Devices technical article describes a motor controller that uses their digital isolators to provide an isolation barrier between the 3 phase bridge/DC bus and the microcontroller. It is mostly about how digital isolators are better than optocouplers, but they do mention this:
The motor power electronics are floating at high voltage potentials while the ADSP-CM40x processor is referenced to earth ground, thus the need for isolation.
I have designed similar circuits, but I always tie the negative DC bus voltage to the same ground as the microcontroller. In this way I can measure the DC bus directly, and my pwm outputs directly drive the FET drivers.
Edit: In other words, I have earth referenced the micro and the DC bus minus. Now the motor works from earth gnd to whatever DC+ is.
Can anyone discuss the advantages of providing this isolation? I realize you would protect your microcontroller if you had some failures in your power stage, but you'd be replacing so many components at that point it isn't likely to matter. Someone could probably also make an argument for not putting all the motor noise into your microcontroller ground, I have seen this problem trying to communicate over usb while driving a motor.
What else you got?