I must admit that, since posting my above comment about the keyboard backlight switching off, I have been unable to reproduce that behaviour ...
I must have been doing something very unusual when that happened - I was messing a lot with my (rooted) chroot setup back then, maybe that can have some crazy side effects.
Whatever. Thanks for clarifying the backlighting logics for me and even pointing me to the code, tdm. If I ever reproduce the bug, knowing how things are supposed to behave will certainly be useful.