Well, flush with my easy success of making the LED controlled with the PWM, I set about modifying the code to drive the motors that way. I changed the relevant digital IO pins to PWM outputs, set the duty cycle in each function that controls the motors, and.... nothing. Nothing at all. Yeah...
Some troubleshooting led me to the chart at https://github.com/ytai/ioio/wiki/Getting-To-Know-The-Board
that shows which pins can be used as PWM outputs. I made a basic error - I connected one of my 4 motor driver pins to pin 8, which is not PWM capable. The interesting part was that it didn't just disable the motor controlled by pin 8 - by trying to set that pin to be a PWM output in my program, it disabled ALL PWM functionality.
As soon as I commented out the references to pin 8, the other motors spun up just fine, and are nicely speed controlled. It's easily fixed - I'm going to swap the motor controller input on 8 to pin 11, and update the code. But it's late, and that can wait until tommorow. :-)