While wrenching away on a customers bike the other day, he was telling me about an experience he had while in the parking lot at the Sage Hen Creek trail head. Apparently a lady had parked there for a quick hike and upon returning to her car found that it had been broken into and her lap-top was taken!!
I hate this kind of stuff. Your best defense is to keep as little as possible in your car and keep it as out of sight as possible while you're out and about. Even a pair of shorts laying on the floor in front of the passenger seat is probably enough to entice some low life scum bag to see if your wallet is still in one of the pockets.
Actually, I guess your best defense would be to ride from your house and just leave the car at home.