About the only thing my rev-a 20" G5 iMac with nv video was able to do in regards to power management for years was to just blank the screen - with the backlight still running.
I glanced over tonight and saw a totally flat-black screen and thought "oh no, there went the display".
To my surprise, the screen is now going to sleep properly (backlight actually turns OFF!) as set in the power management settings!
I'm stoked. I never thought this would work. I honestly don't know if it was an update to Hardy or if I have just not been looking for it.
UPDATE: I thought I'd press my luck and see if putting the compuer to sleep in power prefs would work - nope, it just locks up the machine. Ok, at least the screen goes to sleep properly as set in power prefs, but I still rely on user-inactivity for unattended shutdown: