Under OS 10.4.5, this settings file is called "com.apple.PowerManagement.plist". I've also found the following:

-- The setting of the Energy Saver prefpane's "Put the hard disk to sleep when possible" checkbox (Apple calls this the Disk Sleep Timer setting) sometimes seems to make no difference--the drive(s) will often spin down even if you don't select this option. But what's really going on, is that the time before spindown will always be a positive number of minutes (sometimes 10, sometimes 180--I don't know why the difference), instead of zero, even if you don't select this checkbox, as long as the time for "Put the computer to sleep when it is inactive for:" is set to anything other than "Never". In fact, if you enter Spindownfix's command into Terminal, while the setting for "Put the computer to sleep" is anything other than zero, it will present a warning implying this (without actually saying it directly): "Warning: Idle sleep timings for "AC Power" may not behave as expected. Disk sleep should be non-zero whenever system sleep is non-zero."

-- If you change the Disk Sleep Timer setting in the "com.apple.PowerManagement.plist" file using the Terminal command that Spindownfix uses, the change will stick through a restart, and will remain zero (if that's what you set it to) even if Energy Saver's setting for "Put the computer to sleep when it is inactive for:" is set to anything other than "Never", and will only revert back to a positive number if you open the Energy Saver prefpane and manually change either of the settings for "Put the hard disk to sleep when possible", or "Put the computer to sleep when it is inactive for:". At least that's the case on my Mac, a Power Mac G4 AGP 500 MHz running OS 10.4.5--I don't know why other people are finding this setting doesn't stick after a restart, though maybe they're going into Energy Saver and changing these settings after restarting.

I use Apple's "Property List Editor" utility, which is on the Development Tools/XCode CD that comes with the OS X installer CDs/DVD, to look at the "com.apple.PowerManagement.plist" file.