My G5 1.6 has never been able to sleep properly. When the system tried to go to sleep, the monitor and power button lights would never pulse, and the fans would always spin up to full speed.
I removed my Canopus ADVC-1394 card today and tried putting it to sleep. Sure enough, it slept without problems. Reinstalling the card made the problems come back, and after removing it again, all is good.
So I've definitely determined that the ADVC-1394 (analog capture) card is the culprit. Is anyone else using this card with success? It never caused any problems that I know of in my G4.