&gt; For some reason, though, we don&#x27;t allow that sort of privacy violations in our actual lives.<p>Nomi is a startup that does exactly this. They use open WiFi networks to harvest MAC addresses of cell phones and track people&#x27;s locations that way.<p>The only way to &quot;opt out&quot; is to register your MAC address with them. (Aside from just disabling WiFi on your phone, of course).