teotwaki
Excelsior!
i was thinking of getting somthing like this for my trailerhttp://www.gpsmerchandise.com/p-19-trailerseal-for-mobile-assets-battery-powered-padlock-remote-container-tracking.aspx
Some of their products rely on a 2.4GHz RF identification scheme that uses antenna-tranmitter-receivers along major highways. Some of them seem to rely on cellular coverage. The writeups seem to be deliberately vague but you would want something like a SPOT tracker transmitting updates every ten minutes. SPOT or something similar might be cheaper and it uses a satellite uplink so no dependencies on spotty cell coverage or antennas that are only on major highways