With advancing computer power, the potential for a run-away surveilance state is an increasing threat. Contact network analysis could bring any degree of guilt by association. Automatic numberplate recognition and, eventually, the capacity for roadside cameras to recognise faces of pedestrians and vehicle occupants create the possibility of tracing every movement we make.
Some degree of surveilance is undoubtably necessary, but if it's to be kept under control public scrutiny is essential. Governments will always be tempted to use power intended for use against terrorists and criminals against political dissent.