Cars with RFID security do have lower theft rates, and it makes sense. This type of system makes getting in and driving off a lot more complicated.
Keyless entry and immobilizer systems work in pretty much the same way. Let's say you have a keyless-entry fob. It's a standard radio-transponder setup: Inside is a circuit board, a radio transmitter, a battery and an antenna. When you get near your car, perhaps 5 feet to 10 feet (a few meters) away, you press the button to unlock your doors. The RFID chip in the fob sends out a code of 40 impulses broadcast on different frequencies. The corresponding RFID chip in the car receives this code and accesses the car's software to find out if the code is the right one. If it is, the doors unlock.
This is called an active RFID system, since pushing the button actively sends out the code, instead of receiving it. The immobilizer chips in ignition keys are also active. Keyless ignition, on the other hand, is a passive RFID system. Instead of the ignition chip sending out the code, the car sends out the code and the ignition chip receives it. Ignition systems have no battery (or a different kind), and they have a lower-power antenna, so they won't broadcast as far. It's an additional security measure.
On its face, the system seems impenetrable: There are billions of possible sequences, and brute force will no longer get the car moving. Add in rolling codes, which are becoming more common -- a system in which the expected sequence changes slightly every time you push the button -- and the options get closer to a trillion. But as with any security system, it's only impenetrable until thieves figure out a way around it. Look at safes and burglar alarms; you've got to update those frequently in order to stay ahead of the robbers. Car RFID systems are no different.
RFID hacking is the most high-tech approach to car theft yet. Using hardware that grabs radio frequency signals out of the air, and software that decrypts it, thieves with time on their hands can steal an RFID-equipped car. In 2005, researchers at Johns Hopkins University in Maryland demonstrated how.