I want to point out this new service (API for you fellow developers) that will return a physical address from the MAC address (aka BSSID) of a wireless router. The service is provided by SkyHook Wireless via a simple XML / HTTP post request. Here is an excerpt and link.
For their API to work it requires you to install a browser extension. Which contains, along with the extension source code (which is fully viewable, for Firefox at least), some compiled c++ code (loki.dll for windows). So what does the proprietary stuff do? It does the actual query to their API. And what does it send? It asks your wireless card to list all of the BSSIDs that you are in range of and sends those along with the signal strength of each.