I would love to be able to turn the button backlight off.
In low-light situations, there are many possibilities to further decrease the display brightness, like JuiceDefender or ScreenFilter, but the button backlight always stays on, radiating like a lighthouse (very useful to see where you're going when you walk around the house with the lights off).
For an eReader, it would make sense that the button backlight has an airplane mode like button.
The perfect option would be to have a setting that timeouts the button backlight. After 10 seconds it can turn off, and as soon as I tap again on any button, the lights go on again and I can use the buttons normally.
This would really make sense if it is associated directly to some kind of trigger. For example, when any application hides the status bar (i.e. all reader and video apps). Or maybe when the brightness drops below a certain value, i.e. 10%.