At first, I thought I would only power the treads, and keep head & arms static. I launched Skecthup and started drawing the sides of the torso. I imported a detailed model from the 3D warehouse, scaled it down to match roughly the scale imposed by the off-the-sheld tracks; then simplified the design. 

Then I started playing around with models of small servo... and was quickly convinced that it's possible to add servos inside the arms and neck without sacrificing the overall Wall-E look.