What can I say? I've never been able to draw anything with an Etch-a-Sketch. I won this one in a Christmas gift exchange some years ago and hung onto it with the idea that someday I'd CNC it.

The stepper motors are 7.5 degree Airpax steppers I got surplus many years ago. It turns out those mounting ears are actually NEMA 23 compatible so this should work unmodified with "square" steppers as well. The 7.5 degree steps led me to make the most aggressive gear reduction I could fit in the available space. They take me to about 0.15mm (6 mil) steps with half-stepping.

The stepper drivers are L297+L298 almost straight out of the appnote. They're built on a piece of copper clad with a combination of dead-bug and Manhattan prototyping. I bought all the parts years ago and never got around to etching PCBs. Those flyback diodes are ridiculously oversized because I didn't know better.

I chose http://reprap.org/wiki/Teacup_Firmware because of the builtin gcode interpreter. That saved me from having to integrate with any desktop software. I ported it to the AT90USBKEY you see in the picture and added support for LUFA-based serial right in the AVR. (I already had the AT90USBKEY or I would have just bought an Arduino!)

It easily goes 8000mm/min (which is how fast the pictured spiral was drawn). Given the torque and the large step size I suspect it could go fast enough to melt the stylus if you cared to.

The gears (as printed and uploaded) have slightly too much backlash. The "exact" fit was too tight so I backed off by 1mm which was way too much. They're derived from http://www.thingiverse.com/thing:5505 .

The spiral was drawn in Inkscape and converted to gcode for Unicorn with http://www.thingiverse.com/thing:5986 . I plan to modify that to make more Etch-a-Sketch friendly output.

Video: http://www.youtube.com/watch?v=6UV05jmrK7k

Print one of the mounting plates, then mirror along the long axis and print again for the other side.

Print two each of the stepper gear and the Etch-a-Sketch gear.

Remove the original knobs from the Etch-a-Sketch (they're just press-fit).

The mounting plates are designed to closely follow the contour of the Etch-a-Sketch. I attached mine by applying substantial amount of hot glue (quickly!) to the plate and pressing it into place. The plate is attached very solidly but I'm hopeful it would peel off with some effort.

Mount NEMA 23 steppers to the plates.

Press the large gears onto the Etch-a-Sketch ensuring that the hub faces inward. Make sure the gear clears the mounting screw for the stepper.

Press the small gears onto the stepper shafts with the hubs facing outwards. Align the teeth with the large gears.

Print two feet for it: http://www.thingiverse.com/thing:6872

Connect the steppers to stepper drivers and some kind of controller. Standard gen3, gen5, pololu, etc would all work fine. Find some 2D gcode (such as you might generate for the Unicorn!) and go!