A man's been arrested after flying from NY to LA with someone else's boarding pass that was a day old. As you'll see (if the link works), he's done this more than once.
http://latimesblogs.latimes.com/lanow/2011/06/man-from-nigeria-flies-to-lax-without-valid-boarding-pass-and-identification.html
http://latimesblogs.latimes.com/lanow/2011/06/man-from-nigeria-flies-to-lax-without-valid-boarding-pass-and-identification.html