I've got a 1.33Ghz 12" PowerBook that recently had the hard drive and optical drive swapped. Following the swap, 10.4 was installed on the machine for the first time (previously 10.3.something), and all updates have been applied.
The issue is that the Powerbook doesn't sleep on it's own. It's set to sleep automatically after 15 min, but does not; it also does not sleep immediately when the lid is closed. It does sleep when you choose sleep from the Apple menu, so sleep is working properly.