Note that the tech is there to do this with the cell radio -- no WiFi required. It's just a lot more expensive and hasn't been widely commercialized yet.

The gap that's left open here, as with many semi-anonymized tracking systems, is joining the data up with some other stream of data that identifies the person. Once cell phones become wallets, that should be easy enough for merchants to do. Just join up the purchase transaction data with the location data. Of course the obvious way to do this would be to simply provide an open-access WiFi spot that required a valid email to access. And don't forget that such publicly-accessible data as your WiFi radio ID is easily put on a database and traded. Over time, using several such matching systems, you could probably build up a 95%+ accurate tracking system of people with their identities attached.

Not trying to over-dramatize this story. No matter what your views of privacy and anonymity, to me it's fascinating from simply a technology standpoint to watch all the pieces slowly lining up. It's like watching a puzzle slowly being fitted together.