I knew a simple project would still expose a bunch of stuff I didn't know! With optoisolators (or a transistor) in place, can I drive the motors off of the +9V pin on the Arduino? What about a capacitor across the wires to the fan? My newbie-sense tells me this could smooth spin-up current demands.
I wish I had a good beginner's set of components on hand, but I will start off playing with a couple transistors to drive the motors. I'd need an NPN transistor, right? Is there any reason current could flow back through it, requiring a diode somewhere? (I'll read up on inductive loads this weekend).