I've created a tool called geoloc which uses publicly available information in the form of WiFi packet headers and Google API lookups to build Google maps of WiFi access points.
Anyone who has Unix knowledge can download, install, and run the tool and build their own maps using kismet or airodump-ng outputs. For more information about this, see the HOWTO.
Please note that I have only plotted locations of MAC addresses as reported by Google for WiFi access points I observed when I was at these two locations. If the locations are inaccurate, this is because the data in the Google database for this MAC address is stale. I also collected client device MAC addresses (e.g. iPhones, iPads, other kinds of smart phones, laptops, etc), but I have not plotted this information nor have I published it any other form (nor do I have plans to).
Please also note that I have not hacked into any systems to collect this information. This information was obtained merely by listening to broadcast WiFi traffic and by using this information to drive calls to the Google location APIs. This is exactly the technique that Google (and others) use to build their database of WiFi MAC address locations and to provide location information to mobile applications.
I am aware that publishing this page and associated tool may scare the willies out of some people. Perhaps it should. I have discussed the implications of these kinds of technological capabilities in other recent posts on this blog. In future posts, I will discuss what options might be needed to prevent potential gross violations of privacy implied by these capabilities.