I know there was a program made a while back that allowed you to plot where those traffic cam things were. It patched into the GPS stream, so when you came close to a saved spot - it would sound off an alarm.
There might be a way to mod that software or run a script when you come close to a wifi location. Instead of plotting traffic cameras (not many in the US anyway) - plot your wifi locations.
Software is called Gatso Hunter - I've never personally tested it.
If you can't find it, I know Mobile-Effects.com has it in their downloads listed under the GPS-Other section.