A quick trip to snopes.com revealed this information:http://www.snopes.com/autos/techno/lockcode.asp
In a nutshell, there are a wide range of e-mails floating around out there with 1/2 truths to out-and-out lies. It would appear this one is mostly false.
It seems that since the mid-90's remote locking transmitters use rolling security codes, meaning that each time you use it, a new code is set. So unless you knew what code the transmitter would use the NEXT time, cloning the current signal is useless. The snopes article does a better job of explaining it.