COol, thanks for the reply.
The difference is:
I come back from sleep mode and the problem persists.
I come back from hibernation and the problem is gone (brightness control works)
And i have (sorry forgot to mention) an ATI mobility radeon hd 4200 card
But, the page did help me because it could be a ati driver
The update was recent enough to make it be a suspect of this problem.
All i can do is wait for ATI's driver update.