I experienced the same problem you and others have described, on both tiger and leopard. My screens' backlight would go off 3 or 4 minutes(it goes dark but you can barely make out the screen). It didn't matter what i did with the energy saver settings, problem persisted.
After searching google endlessly and reading what others have to say, I came across a solution that worked for me.
You start off by deleting your energy savings preferences. Restarting your machine. Set your energy savings to better performance THEN using terminal, you set your sleep to 0, autodim to 0, disksleep to 0, and displaysleep to 0. You might also disable your screen saver. I have mine for 15 minutes.
Do not mess with your energy settings anymore.
The drawback to this, is your screen will NEVER go to sleep and according to apple, "never letting your display sleep may shorten it's life"
I always just close the lid on my machine, so this isn't an issue. I also have a shortcut key(thanks quicksilver) to sleep my machine. I haven't experimenting with setting the sleep to something like 30 minutes because everytime I do, the problem returns and I have to do all of the previous steps again.
conclusion
I think a lot of it had to do with corrupt energy saver settings and possibly a bug that corrupts your settings if you mess around with the energy savings settings. The workaround as I described is trashign the preferences and changing the energy saver settings via terminal.(Turning off sleep, dim, sleepdisplay)