Please Rob Me uses Twitter and Foursquare to tell you who's not home
The point isn't to have people actually robbed - there's a disclaimer at the bottom of the page that makes that clear - it's to point out the potential security hole people are opening by telling a collection of strangers that they're out of the house. This only gets worse when your friends add your house as a Foursquare location, so your address gets tweeted every time some annoying-public Foursquare buddy checks in there.
Do I think this is an enormous security risk? Not really, but there have been documented - although probably exaggerated - cases of people being robbed after announcing their vacation plans on Twitter. Telling folks where you are isn't the hugest risk if you keep your address fairly private. It's like installing The Club in your car: it won't keep someone from stealing from you, but if your home address is a little harder to Google than the next guy's, these hypothetical robbers will move on and pluck the low-hanging fruit ...
... well, unless someone who knows where you live has a personal vendetta against you. Can't really blame Foursquare for that!