For this trick I am about 90% sure that it has to be working enough for you to read the correct resistance that the ignition unit sends out. If it's failed beyond all hope I believe you must buy a new lock. I've read about some tool that GM has that you can stick your key in and find out what resistance value the ignition cylinder would spit out with that key and then you could try that resistance but I'm not sure if that's fact or fiction.
Basically I'm saying that you'll probably have to buy a new ignition cylinder but when ponchonutty sees this he might have some better advice for you.