Good writeup about "skyhook" technology, which had a bunch of their workers drive around the country and marking down the locations of umptity umptity umptity 802.11 signals...
"... Metropolitan areas today are blanketed by overlapping Wi-Fi signals. At a typical Manhattan intersection, you might be in range of 20 base stations.
"Skyhook's big idea: If you could somehow correlate those beacon signals with their physical locations, you could pinpoint your own location, G.P.S.-style, but without G.P.S.."
rest: