Hey all, so I'm having this same issue, I'm using Smoked Glass 5.0.1 ROM, here is what I've done and whats happening:
When I push the lock/unlock button, the capacitive buttons will turn on for about 1.5 seconds, then turn off, when I then unlock the phone, the backlight is off.
When I turn on/off the auto-dim function, the capacitive buttons light up, and stay lit up, but as soon as I slide my screen to use the keyboard, they turn off, and the keyboard lights are off as well.
When I toggle the backlight control on the power settings widget (while the phone is closed) the capacitive buttons light up and stay lit up, when I slide the screen, they turn off and the keyboard lights are off as well.
When I open the physical keyboard, and do any of the above steps, the keyboard and capacitive button lights turn on and stay on, until I close the phone, then they all turn off again.
I couldn't get them to come on at all before now at least I have a remedy when I need to type in the dark instead of guessing, but does anyone have a permanent fix for this yet?