This evening I got my drawbot up and running for the first time. The mechanics are still pretty shakey, but the electronics have been burned in for several hours now and I'm pretty confident they're solid. The 'bot is based on an Arduino with a dual axis opto-isolated unipolar stepper motor driver shield that I designed and built myself. As soon as I can figure out a place to put them, I'll post the Eagle files for the shield.
I'm writing a PC application in Python to do some image manipulation and drive the 'bot. When I get it further along I'll post again.