lcd luminosity keys disabled

[Pierre Haessig <pierre.haessig@crans.org>]

Hello everybody,
For a couple of weeks (probably after an update as I'm using testing) I
cannot setup the luminosity of my HP laptop's screen : the dedicated key
combinations ("Fn" + F7 and "Fn" + F8) don't do their job anymore. I
suspect they are intercepted somehow by a program (I'm using gnome 2.18
and xorg 7.3) but my knowledge of X key binding is very poor. I didn't
find a similar problem with google. Still I particulary suspect gnome
because it can usefully intercept for example "Fn" + F3 (= "www" key
recognize by gnome as "0xb2") to launch iceweasel. 
Does anyone have some  clue to enable these keys once again ?

Thank you for your answers and please forget my english mistakes.

		Pierre H.