My rover base and motor controller arrived, and I'm quite pleased with them. The rover base is larger than I expected and very solid. I've gotten as far as hooking up the controller to the base and manually jumpering +5V to the control pins to get the treads to spin and confirm all four channels of the controller are ok. Everything looks good.
The motor controller has two inputs per motor - a pulse width modulation input and a direction input. Each motor thus requires two inputs.
However, two motors drive each tread, and it's a safe bet that the motors on each side will be running at the same time and in the same direction, so I plan on driving each side of the vehicle from the same two IO pins.
cool little logic level shifter boards from Gravitech to convert them. Mouser Electronics carries them, and I got it there. Odds are it would have switched at 3.3V, but in the interest of reliability I decided it was worth the effort. I did look at the shifters from Sparkfun, which are small and inexpensive, but this one is easier to mount to the chassis, so it won.
Next step is to add the needed few lines of code to the server program that runs on phone to actually toggle the IOIO pins in response to client input, and verify I actually get 5V out the other side of the logic converter. At that point I'll be ready to test the motor control over the network. More to follow.