Out of curiosity -- how many of you use EZ-Pass on the highways?
Otherwise ironclad-appearing alibis have been demolished in court by EZ-Pass records ...
I haven't heard of it as much, but cell phone records could be used to do the same. There is nothing technical stopping the telecoms from recording and timestamping what cell phone tower your phone is currently associated with. The reason why cellular phones are called "cells" is because coverage is broken up into discrete areas. When your phone enters a cell, it contacts the towers to authenticate and register itself. Equipment in the tower tells the rest of the phone network that said phone is physically located in this particular cell, so route all calls or data to this particular cell. This isn't a bad thing, it's pretty much just dictated by laws of physics.
However, if these authentications/registrations were recorded, it would give a pretty accurate description of where a person was over a very long period of time.
All phones must include GPS chips by such and such date. For your safety, of course. But if someone hacked their GPS chip, the cell repeater information would still be valid. Actually, it'd be handy for identifying people who hacked their phones. If requested GPS data doesn't match up with tower info after a couple of requests, viola, you have successfully identified someone violating FCC regs.
Just remember, always remove the battery from your phone instead of turning it off.