That's a pure awesome device... I'm in the process of building something sort of similar, a suspension wall plotter.. two steppers instead of three, but otherwise pretty much the same, in that the head is suspended from a variable length system under stepper control.. http://arduino.cc/forum/index.php/topic,60281.0.html
How are you compensating for the difference in distances radially (in that a movement toward the center is "smaller" than a movement further away)?
I'm currently working on calculation code, which will determine location by using pythagorean theorem and then apply a "correction" to the movement, but it's not quite there yet. Your device (especially since it used three motors) is MUCH faster than mine, if you operate the steppers at a slower rate do you get increased accuracy? My largest problem is of course being suspended, it's got a bit of bounce when it moves.
In any case, TREMENDOUS job! Sweet implementation, it's quite a nice looking device on top of being functional!