License Plate Scanners and Privacy in Public Spaces

Here is an interesting article about the powerful abilities of automatic license plate scanners being used in the United States.  A single scanner is capable of photographing and cataloging (location, time, date) 1,800 plates per minute, regardless of speed or driving conditions.  The scans are typically updated to one of a few enormous central databases, such as the Digital Recognition Network (DRN), which claims to add 70 million scans per month.
Some states are considering enacting legislation that would ban most uses of these scanners, perhaps making exceptions for police and parking regulators.  Much of the private sector that currently uses the scans, like insurance companies, repo companies, and private investigators, strongly opposes the legislation, arguing that they are not invading privacy by photographing something which is available for all to see.  The debate raises some interesting questions regarding expectations of privacy in public spaces.