I agree it's definitely easier to do - and obviously already happening in this context. Also billboards that use Kinect CV to record emotional response. [1]

I was just intrigued by the perceptions around this - people often have an instinctive strongly negative response to the idea of pervasive face recognition but I'm not sure they would feel as unnerved by the presence of a wifi network with this sort of "surveillance" traffic running through it. Maybe they would.

[1] http://theconversation.com/now-advertising-billboards-can-re...