From the site...
This new procedure uses wireless network data collected from walking and/or driving scans, aerial photography, and interpolation techniques to create highly detailed network coverage and signal strength maps.
This example image represents the signal emitted from a single wireless access point located in downtown Lawrence, Kansas. The signal strength degrades from the stronger values in blue to weaker values in orange.
I particularly like the detail and the watercolour effect they have. The data does not obscure the underlying map, but merely adds another channel of information. I also think this could be done continuously as a little background process on your laptop+GPS that updated the server whenever it manages to reconnect. With enough people doing this, you could check the status of the network before you leave the house, or perhaps to find a nearby location with better signal stength.
One problem is that they fake the spread throughout the map. They use InverseDistanceInterpolation to guess what the stength is in a radius, but it's not going to be very accurate in, say, downtown where a lot of skyscrapers abound, or near a electricity transformer yard. -- SunirShah