Ok. I found my problem. Was a kernel bug. It should be fixed in 2.6.32.
See
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=d822d5c273683dc4bacd413953b11ad31513e997
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[Karmic] dim display when idle only dims but doesn't restore the previous
value when not idle
Same here with an Asus EEE 1000h. It seems like the BIOS sets the
backlight direct and does not inform the system about it. xrandr
--prop detects the correct backlight value but gnome-power-manager
seems not to be aware of it. There is also no notify osd while changing
the brightness.
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