But it is clear that Apple, Google, Navizon and Skyhook are collecting MAC (Media Access Control) addresses, which can be used to identify wireless routers. They are also collecting data about the network's signal strength and then linking the Wi-Fi data with other information, such as cell tower and GPS readings, to get a very clear idea of where their users are located.

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After Google Incident, Wi-Fi Data Collection Goes on
http://www.pcworld.com/article/205062/after_google_incident_wifi_data_collection_goes_on.html