While this is much simpler than Mike Park's PropEtcher (see http://www.youtube.com/watch?v=MgyZgqImAHA) it was as much a test of my CNC machine cutting acrylic as it was making an automated Etch-a-Sketch.
The circuit is incredibly simple: it's a Protoboard and a single darlington array to control a pair of unipolar stepper motors (the ones Parallax sells here: http://www.parallax.com/StoreSearchR...5/Default.aspx)
I used my home-built CNC router to cut the face plate and gears from a single piece of acrylic and replaced the existing red frame with my new CNC'd one.
The software is written entirely in SPIN and supports a simple "goto" function that takes an X & Y coordinate. It uses a Bresenham line algorithm to control the step rate of the motors. With gearing and using 1/2 steps, the usable screen area is about 800 x 600 units.
Here's a video of it in action: http://www.youtube.com/watch?v=W3iPjJfIIOs
...and another of it drawing the Hack-a-Day logo: http://www.youtube.com/watch?v=-MNqW1GT4nc
Code is attached.