Actually, catalytic converters are pretty expensive, and their theft is on the rise. I know someone who had his car stolen in Oakland. The police found the car, but the catalytic converter was sawed off. Supposedly they run a couple hundred $ minimum.
Would like to read that article, but its probably a few bits of anecdotal evidence spun into a trend and story. Also I am lazy and forgot my NYT password.
* http://www.nytimes.com/2008/03/29/us/29converters.html