My box of parts has shipped from Solarbotics, so it's just a waiting game now for the motor and other bits. I was ready to cut the 1/4" plywood but decided to wait until I get the motor and determine that it can actually be mounted properly, including the hole I'll have to drill for it to drive a wheel.
In the meantime, I took a look ahead to the next robot that will require two photoresistors and two servos. I didn't find what I needed at Solarbotics, so I'm going to keep looking. One thing I'm finding is that servos on a lot of electronics websites come with almost no documentation or so much that you can't make sense of it. For those of you who have not yet started in on Chapter 5, just be warned that there's a fair bit of hunting involved - many of the key parts don't come in either Make: Electronics component kit 1 or 2... makes me think Make might want to consider a final kit titled "Chapter 5: The Missing Bits."