Recently I've upgraded to Lion and in general I'm not satisfied with its performance, but what's really annoying is that the login screen always sets the keyboard backlight to the max (even when I set it to zero, after re-entering login screen, it's back to the max level).
That's not quite bad, however that maximum level of the keyboard backlight suppresses my setting (zero highlighting) when I log in. So when I log in, the keyboard backlight is still very bright, as it was at the login screen, I press F5 to discover that OS thinks the keyboard backlight is off. Pressing F6 brings the backlight down to 1st level, so pressing F5 again, turns the keyboard backlight off.
It was persistent at 10.7 and continues to be so for 10.7.2.
UPDATE: since some time I didn't see it. I'm running 10.8.4.
Best Answer
I don't know if it's Lion or a hardware feature, but on my MacBook Air, the system changes the keyboard lighting (and screen brightness) depending on the ambient brightness, which is detected by a sensor near the iSight lens at the top of the screen. In bright surroundings, the keyboard lighting is disabled and pressing F5 or F6 does nothing.