First a quick disclaimer - as far as RepRap is concerned I'm a total lurker - I've been hanging around the site for a while because the idea is just so totally awesome, but I've not done any work on a Reprap or repstrap so I don't know a whole lot about the details of their electronic and software workings (and have no way of testing crackpot ideas like this one). On the other hand I have do have some experience with the Arduino and running out of pins, and with steppermotors and drives (and, in fact all of those at once - built a little robot/electic vehicle thingy for Science Olympiad last year using a step motor and arduino board).
Anyway, I just had a little thought reviewing the connection diagram and thinking about everyone complaining about not having enough pins on the arduino electronics: what if you tied all of the direction pins for the stepper motors together? It'd slow down the head movement and complicate the firmware a bit because you could only move one axis at a time, but if you desperately needed the pins for something it might be a useful hack.
