well i've seen it turn off on some computers, but I have never been able to get mine to turn off completely under dim lighting conditions. Apple didn't bother to get drivers right somehow and a on some of the laptops, the backlight just doesn't turn off completely, whereas it does on some others (I've seen it turn off completely on some of the older Santa Rosa models, but it doesn't turn off completely on my early 2008 MBP.)
Turning the backlight on the keyboard down "all the way" on XP still leaves the backlight on the keyboard, just less than if it was turned up all the way. You can dim it, but you can't get it to turn off completely. At least that is my experience, and I know there are plenty of other XP users that have experienced the same problem with bootcamp drivers. (I've done multiple fresh installations over the time that I've had my computer and not a single one of those fresh installations fixed this problem, so it's pretty clear that it is the Apple bootcamp drivers that are to blame and not something else.)