Here's an example that prints out the viewpoint velocity each frame:
Code:
import viz
viz.go()
viz.addChild('piazza.osgb')
lastPos = [0,0,0]
def calculateVelocity():
global lastPos
pos = viz.MainView.getPosition()
distance = vizmat.Distance(lastPos,pos)
velocity = distance/viz.getFrameElapsed()
print velocity
lastPos = pos
vizact.onupdate(0,calculateVelocity)