It's going to get a lot worse.

Part of the specifications for the future ODBC (Car emissions/control computer) called for both wireless and IPv6 addresses. The idea for those states with emissions controls, is roadside scanners which read your car's ODBC remotely. If your have an emissions code, the system would report it to the road-side system which would forward it on to the authorities. You get a 60 day "fix or it fine" card in the mail. In the lab, computer researchers have already demonstrated vulnerabilities in these systems.

In a few short years (2011 or 2012 model year), all new cars for sale in the US will be required to have radio based sensors in the tires to measure pressure and report it to the driver. These are unencrypted systems. Station A can scan the tires and OCR your plate; station B can be a cheaper one that just scans your tires. Only the manufacturer knows which tires go with what VIN, but that's just a legal triviality. These systems also have been shown to have no security.

At least the toll transponder system is voluntary, but that too has been used in legal cases (murder, divorces). Locally one transportation research group uses these to measure road speed -- they upgraded from doing license plate OCR with cameras. The camera system was also reported to be able to count passengers in the car (presumably as a means of determining carpooling) but I was never able to verify that function existed. The cameras are still present where they added the radio receivers.

There's reports of legislation proposing that RFID tags be part of each vehicle's license plate. It's not that that can't pick up your plate via OCR, but it makes it a lot more reliable with RFID and works from more angles. There's also a more extreme proposal, where via two way communications, your engine could be shutdown ala On-Star, but it would be required for all new vehicles.

Municipalities have purchased systems that scan license plates at a high rate from a vehicle that drives by. They are looking for tax and parking ticket scofflaws for revenue purposes. Stolen vehicles / plates can be programmed in as an alert, but it's only a matter of time before they just scan and record, and sift the data later.

In those places with red light cameras, there is a legal assumption that the registered owner of the car was driving it, until and if they prove otherwise. These High School kids printed their teacher's license plate, used a car with the same color and model, and ran a few stop lights. If one hadn't confessed, the teachers was screwed. The plate became a proxy for the person. In the future your car shows up in a funny place, you may have some explaining to do, even if you weren't driving it.

The expectation of privacy in where you move around was gone years ago. It may be possible to get that genie back in the bottle, but it'll be far from easy, with the goal being 'security', fighting 'crime' and government revenue.