We already built a board capable of echoing back serial input. Now, we're rebuilding that board from scratch in Eagle, a circuit board design software package, and adding our own components.
From an image of a 1-sided PCB, it's easy enough to duplicate a circuit.
Programming the board went smoothly. I wrote a C program (really just modified Neil's slightly) which waits for the whole phrase "turn on the light please" to be typed into the Echo python window before it will light up the blue LED.