Well, the fun is increasing with Arduino.  We hooked up some circuitry which enables a transistor to switch a fairly large 12V load via a very small 5V digital pin on the Arduino board.

This has been a great learning resource: http://www.kpsec.freeuk.com/trancirc.htm

Basically, we have 12V power going through a motor circuit (protected by a signal diode), and then to the collector of a transistor.  The emitter of the transister is connected to ground.

Lastly, the Arduino ground is joined with the emitter, and the transistor is switched by a 5V digital output pin on the Arduino board.

Now, just need to find something useful to do!