iPhone & iPod touch use a service called Skyhook that can track your position by detecting what WiFi signals are around.  Skyhook sent out roaming vans and associated WiFi signals they detected with specific locations. Pretty dang cool. Looks like they have a developer API as well. Doing some quick searches I also found similar services (Herecast) that could potentially be used to extend functionality of other WiFi enabled devices.

Related:

http://machinesthatgobing.com/apple/skyhook-macworld-apple/