FWIW I hav e been testing my MBP's battery life and the keeping the keyboard backlight on costs 15-20 minutes of run time. It does have a light sensor so it is only on when the ambient light gets low enough. You can adjust the intensity too.
The keyboard light seems to be independent of the others. It usually dims be fore the screen and well before the the screen dims. I'll pay more attention to it. The light sensor for it is in the left speaker grill by the magsafe. When I cover it the KL comes on.
Sorry I was wrong I just tested it on this post and they dimmed together. The weird thing is the KL was dimming earlier on it's own, unless the ambient light was on the border of activating it.