From Wired News (as pointed out by Schneier):

The British government is preparing to test new high-tech license plates containing microchips capable of transmitting unique vehicle identification numbers and other data to readers more than 300 feet away.

Officials in the United States say they'll be closely watching the British trial as they contemplate initiating their own tests of the plates, which incorporate radio frequency identification, or RFID, tags to make vehicles electronically trackable.

"We definitely have an interest in testing an RFID-tagged license plate," said Jerry Dike, chairman of the American Association of Motor Vehicle Administrators and director of the Vehicle Titles and Registration Division of the Texas Department of Transportation.

...

Proponents argue that making such RFID tags mandatory and ubiquitous is a logical move to counter the threat of terrorists using the roadways, and that it will scoop up insurance and registration scofflaws in the process.

This scheme fails my Jews in the Attic Test and must be vigorously opposed.  Besides, terrorists will just destroy or otherwise neutralize the transmitter when they go on a mission.  And if detection and sanctions are in place for destroyed devices they will just destroy the devices on vehicles at random in the shopping mall parking lot until the population at large is fed up with the harassment and the police effectively ignore destroyed devices.  It's costly, it can't accomplish it's intended goals, and it's a hazard to freedom.  Don't even bother testing it.