The technology, from Portsmouth, England-based Path Intelligence, is called Footpath. It uses monitoring units distributed throughout a mall or retail environment to sense the movement of customers by triangulation, using the strength of their cellphone signals. That data is collected and run through analytics by Path, and provided back to retailers through a secure website.
Apparently the service can then use the data to create a heatmap of the store. While creepy, this is also pretty cool.
Also: time to bust out the tinfoil hats.