I have been testing Ubuntu 12.10 on my Thinkpad T420 (with nvidia optimus, yes) and over the last couple of days I've made 10-20 clean installs. Every install I've (until now) has had the brightness hot keys working out of the box. But this morning I've made two new clean installs (using the same media as always) and not the hot keys do not work out of the box. I am able to adjust the brightness by going to "System settings" -> "Brightness and Lock", but the hot keys (Fn + Home/End) simply does not work. I have installed nothing but bare Ubuntu (no nvidia drivers, nothing). I am certain this used to work for me (clean install, hot keys working of the box) because I ran a series of power tests using "powerstat" on both clean installs, installs with drivers, installs with bumblebee etc. and they were all made with minimum brightness - hence they hotkeys have worked for me on clean installs just a couple of days ago.
What could have gone wrong and how to I start to debug?