For this particular problem, you could probably just stick
echo 1000 > /sys/class/backlight/intel_backlight/brightness in
/etc/rc.local (before the
exit-line). That would run it on boot, as root before the desktop was loaded.
But in general (how do I run a script as root?) things are a bit sticker...
You could just run
sudo ./myscript (by editing
sudoers) but just allowing
./myscript to be run without a password would mean you (or a program running as you) could edit that file and suddenly you can run anything as root without going through the proper procedures. Uber-poor security.
Instead I would suggest:
sudo mv myscript /sbin/myscript
sudo chown root:root /sbin/myscript
sudo chmod 700 /sbin/myscript
ch* commands now mean only root can read/run the script. Sticking it in
/sbin/ is just incidental - you don't have to do that.
Now you need to edit
/etc/sudoers. The Wiki has a good example and you'll just need to substitute the shutdown commands with your script's path (use a full, absolute path).
NOPASSWD is the key keyword here. It allows you to run something as sudo without the password. So then you can just add
sudo /sbin/myscript to your startup jibberyboo and you're flying.