SkyHook Wireless has been wardriving the US for years creating a huge database mapping wireless routers' MAC addresses to their physical locations. They provide an minimally documented API (doc here) which allows anyone to query the database directly for any MAC address. This could potentially allow some malicious individual to find out exactly where you live. Of course for them to get the MAC of your router in most cases will require either being infected with malware or some sort of social engineering attack... Imagine if you got a phishing email that included your home address.