Don,

You may be interested in some maps I’ve come across which plot the location of wireless computer base stations in two US cities. A company called Skyhook Wireless has made these maps because they are using the base stations as an alternative to GPS positioning.

They give an indication of how widespread and inescapable these devices are:

http://www.skyhookwireless.com/images/santamonica.jpg
http://www.skyhookwireless.com/images/nyc.jpg

Regards
Barry Brannan