Android's USB port to control motors

I have been thinking for quite some time about a useless but fun project, basically a phone case with wheels, controlled by your phone. I however don't know how to make about everything between the motor drivers (most likely a H-bridge) and the USB. I would like to avoid putting the oh-practical but big arduino in my design.(I ask it here because I am not sure where else I can go) Any ideas?

Use an Arduino micro. That is a small form factor and will connect direct to your USB port. Google for a h-bridge chip their are lots about.