It seems to me the take-away of this discussion is the use of the servo.write functions to easily get up and running with an Arduino and a servo. The servo has 3-wires going to it. 2 are just the +5V power and ground that the Arduino has anyway. The 3rd is a single digital output pin. The single servo.write(0) command sends the servo to one end, the single servo.write (180) sends it to the other end. And arguments in-between send the servo to some angle in-between. Easy.
Then build or buy a modified servo which has come to be known as a Continuous Rotation Servo and now the same commands send the mechanism spinning in one direction or the other direction and coming to a halt by sending the single servo.write(90) or mid-value command. Easy.
If you are going to mess with servos, I highly recommend the $1 servo-tester you can get on eBay. Simply apply 5V DC. Attach 1 (or up to 3) servo mechanisms via the standard 3-wire connector. Then by turning the knob on the exerciser, you effectively generate servo.write(x) commands so you can verify operation of your servo mechanism before writing a single line of Arduino code. This development tool is obviously handy when mating your servo to your animation.
Again, I think the idea here is to get up and running quickly with minimal fuss. That's what is interesting about the CRS since it accepts and interprets Arduino servo.write commands. As I've already confessed, I was not aware of this development until Leo pointed it out. Anyway, to further the discussion about using servos for layout animations, here are a couple more ideas that might help Arduino enthusiasts willing to modify (or pay a couple bucks extra to modify) a standard servo as is done with a Continuous Rotation Servo.
Watch the 4 mechanisms in action (one being an un-modified standard servo).
In photo (and video), I modified the "guts" of a standard servo to make a CRS. As mentioned in a previous post, I then replaced the DC motor within the CRS with a cheap DC gearmotor. In fact, I show two DC gearmotors being driven together by a single CRS. The point here was a standard servo has pretty amazing current drive capability since it can spin the integral motor at whiplash like speeds. But if you want to go slower as you might with many layout animations, there is plenty of current capability to drive a wide variety of DC motors.
Additionally, on the top, I show a "linear" servo which is an incredibly useful mechanism for layout animations. Sometimes you just want something to go left-to-right rather than around-and-around.
Again, it's just a modified standard servo removing the electronic guts and attaching it to a linear volume control mechanism from a remote-control audio amplifier - you've seen them...when you adjust volume from the remote, the manual volume control knob slides (or rotates) by itself. Or you can just go up to the amp and manually slide the volume control. It's essentially a motor-driven potentiometer. So the potentiometer provides the feedback to the servo electronics. I buy these surplus for a few bucks. Again, you have the standard 3-wire connector (+5V, ground, 1-pin Arduino output) so from the Arduino perspective, the single command servo.write(0) sends the slider to one end, servo.write(180) sends the slider to the other end, and value in between send the slide to the mid-point. Easy.