every time you burn a disc a digital signature similar to your a serial number is left on the disc. rental stores periodically check digital signatures on their discs and report them to the police. don't be surprised if they come knocking down your door if your name comes up too many times
that's what you should have told him!
[all information contained in the above paragraph is false and is intended as a show of my disgust toward anyone that would post asking about burning rental dvds...GRRRRR]