On re:publica 2013 the Berlin based german data designers from OpenDataCity created a wifi tracking network with 100 Access Points that allowed them to visualize the movements of about 6,700 different electronic devices during the conference.

The application called re:log is a dynamic map of the conference location that shows the approximate locations of the devices when they were connected to the local WiFi hotspots. An interactive timeline underneath allows to explore the dynamic changes over time, while a rectangular area can be drawn to more specifically highlight and follow a smaller amount of dots.

The visualization was based on tracking the MAC addresses of the devices according to the WiFi hotspot they were connected to. This data, which can be downloaded, was fully anonymized, yet the authors mention their desire to allow people to look up their own MAC address in the future.

I suspect the solution used is based on MagicMap a free Wifi/Bluetooth tracking architecture developed at the Humboldt-University Berlin. Their Wiki has some more information.