Lojack is a device placed in cars that tracks them using GPS should they be stolen. Because it is hidden but effective, Ian Ayres and Steve Levitt estimated that even having a small share of cars with Lojack installed has a disproportionate impact on the amount of crime in a city.
I wonder if the same effect is happening with some Apple products through MobileMe. If you a MobileMe subscriber you can use the Find My iPhone feature to locate your lost phone. Already there are reports of this aiding in finding stolen iPhones — here and here. But this also works for laptops too. Today a story about someone using the Screen Sharing feature and observing a thief using their stolen laptop to apply for a job and getting their details as a result. Aside from the fact that this is a laudable activity for a criminal, if this type of story became more widely known, then would-be thieves would be deterred from stealing Apple products in general. That said, it would probably be good if Apple made it harder to turn off the MobileMe service once activated.