Hi Jeremy,
viz.translate(viz.HEAD_POS) is mostly there for backwards compatibility. It will translate the head position relatively to the current position.
viewpoint.translate will translate the head position either absolutely or relatively, depending on the optional fourth parameter. (The default is absolute)
viewpoint.move will translate the head position relative to the current orientation. So the following will move the viewpoint forward by 1 meter, no matter what the orientation is:
Code:
viewpoint.move(0,0,1)
Hope that clears it up