Triangulation based on signal strength, not too hard actually. Math is op. If you're worried then try to blend in with a lot of other users.
edit: theres a video somewhere of a guy who setup a city wide wifi network of sorts that would sniff the air and track cellphones and laptops movements accurate to about 10m or so. This coming from some bored creep. Cant be bothered to dig up the talk right now, I wanna say it was from the most recent defcon.