On my Dell Latitude E6400, the minimum brightness level seems to be set to an incorrect and way too low level; the lowest brightness setting (both via the Fn+Up/Down keys and the control panel/mobility center) totally turns off the backlight, where I would expect it to still have a slight backlight.
The problem is that this messes up the adaptive brightness service; when the light sensor doesn't detect any light it sets the backlight to the lowest level, except the lowest level is a total absence of backlight and that makes the adaptive brightness totally unusable.
Edit: the adaptive brightness problem is (hopefully) solved with this ALR curve, but the question still stands - it shouldn't be possible (nor useful) to lock yourself out of the machine by not being able to read anything on the screen (the Fn keys aren't reliable enough and sometimes don't work after standby, thus you need to blindly open up the mobility center and set the brightness back up a bit).
I have the latest BIOS (version A34 if I remember correctly), and I installed the latest Intel display drivers, nothing changed. I'm looking for a registry hack or something that can change the lowest brightness level to something higher, where the backlight will still be active.