Duh.  Basic idea of what I described has not only been done, there is even a name: wardriving.  I remember reading about a guy who did that in NY sometime back, but didn't connect the dots.  Duh Again.  I also found several sites where WiFi maps can be found.  Here is a national one and here is site where more detailed maps can be found although its SF area map shows WiFi points offshore.  Are these warboats or errors?

While GPS-based solution is done, I have not yet located one that uses addresses entered by WiFi station operators.  I did find an address to GPS coordinate mapping servce.  Wow!  This means all I need is a bit of software and lots of publicity to get WiFi Worldmap project off the ground.  Cool!