Thanks for the feedback. I really liked building this one, and it's only going to get better.
I used the motor from two Roombas a friend gave me. They both had wear and tear problems but the motors worked fine.
I'm not using the wheel encoders as of yet. I've taken the wheel apart to see them and it's a simple broken IR beam setup. Very hackable.
I salvaged lots of cool Roomba parts....
I have four PWM outputs from the Arduino going directly to the h-bridge on the Roomba main board which is mounted in the prototype with some duct tape. The battery is plugged into the board but it doesn't need to be turned "on" to work. The power is at the transistors all the time. All I'm doing, is triggering the smaller switching transistor that turns on the power transistor that runs the motors. Since I'm doing this with a PWM signal, the motors are running at full torque no matter what speed they are being told to go. I can go forward and reverse so I can "tank steer" it.
It's surprising how easy it is to do this h-bridge hack on just about anything with a motor control on the PCB. Just look for groups of four power transistors and start probing smaller transistors on the base with 5V+ until the motor runs!
It's a bit seat-of-the-pants but when you know how to read a circuit it doesn't take long and I've yet to damage anything.
I'll be working more with the sensors and main board from the Roombas...