For me, the LED lights turn off when I go into Battery Saver Mode (at 20%, I think). When I plug in they come back on, and when it recharges above Battery Saver it is on. Also, the ambient sensor must deactivate the LED in bright conditions, because mine don't turn on in direct light, but turn them off and they come on. Hope this helps!
I have noticed some weird behaviour with my LED lights for Back, Windows, and Search buttons; sometimes when I turn on the phone they turn on and stay on, and sometimes they just don't turn on and stay off even when I touch them.
I don't know how its suppose to work on WP8 or Lumia phones, but on my Samsung Focus it works that if I touch any button it lights up briefly and then turns off after few seconds, isn't that how its suppose to work?
I am trying to work out what is the normal behaviour for the LED button lights on WP8/Lumia to determine if there is an issue, because it definitely works differently then on my old WP7.
Update:
Tested the power saver setting, when power saver is on buttons LED is off, but when power saver is off the buttons LED is on all the time, even when watching videos which is a bit distracting, any way to make it behave like on WP7, so the button LED only turns on when a button is touched?