If you set your Mac's screen brightness before restarting for Firewire target disk mode, the Mac remembers the setting when it boots up in the mode. So if you want to conserve battery power or backlight life on your portable when you connect it to your desktop, you can turn off the backlight before you restart and it will stay off.
If you elect to turn off the backlight completely, make sure not to hit the power button accidentially. It would be possible to forget that the unit is in disk mode and then hit the power button to turn it on. But since it's already on, the power button would turn it off without unmounting from the host unit and probably corrupt data. So be extra careful in this case.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040626154219390