Is it possible to find the physical location of a wireless router based upon the broadcast signal?
Solution 1:
As far as off-the-shelf solutions go, many commercial 802.11 networking infrastructures support triangulation of both host computers and "rogue" APs by comparing the signal strength of the source among all the APs that can see the signal. I've used trapeze wireless systems that import autocad maps that include things like building materials (to compensate for different signal attenuations from drywall vs brick vs steel reinforced concrete). It then draws a picture that has all stations and APs. I believe cisco and aruba have similar tools.
You can't do it with one or even two observing stations, but you could probably do it with a mobile observing station and GPS. I'm not sure what software there is to do such a thing but the usual suspects such as netstumbler may well support such a feature.
Solution 2:
You can do some basic Radio Direction Finding to locate the signal.
At 2.4 GHz it is easy to come across antennas with small beamwidths. Sweeping one around while watching the signal strength should give you a good idea of what direction the AP is in.
Do that from a few physically separated locations and you will have the beginnings of a triangulation. It doesn't have to be super accurate to get a good idea of where the transmitter is located.
The faster your signal strength indicator, the better. On a radio with an analog meter, it's amazing how fast you can "walk down" a transmitter with a modest yagi antenna.
Solution 3:
By itself, there is no way to get that information from a wifi signal. Google, however, has indexed wifi access points with their locations. So they are able to tell where you are based on the wifi AP you are connected to. In a case like that, it is possible to get a location based on wifi.
http://googlemobile.blogspot.com/2008/10/my-location-now-with-wi-fi.html
Solution 4:
If you've got an android phone try OpenSignalMaps, click on the wifi/cells toggle on the bottom left to see the wifi info. Shows the positions of wifi routers on a map.