What are wi-fi connections revealing about you?

I work in the Wi-Fi sector. It's amazing how much data clients can and will collect and use for retail/advertising purposes. Chances are whenever you see a 'Free Wi-Fi' sign your presence and dwell time (at the very least) are being recorded.

High-end systems can track an individual and follow them on a map, noting things like which stores they visit at a shopping center. The sheer volume of data collected is fantastic (from the perspective of a data-nerd like myself) and scary at the same time.