I've just upgraded to Panther, sorry if this is also true in 10.2.x. I saw this on my TiBook 500Mhz.
In the Energy Saver preferences, if you set your display to sleep in 1 hour (for example), it will sleep in 1 hour ... but it will dim in 30 minutes! As far as I can tell, you are stuck with the dimming time always being 50% of the sleep time. The only way to see this is to see the pop up info over the Display sleep slider -- hover over the slider, and you'll see "Dim 30 minutes. Sleep 1 hour."
Since the dimming interval is fixed, if you want to maximize your battery life by dimming the screen quickly, you'll also have to set a reasonably short time for putting the display to sleep.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031130211651351