First of all, a big big thanks for your work! It is so helpful!
I have a Samsung R580
with NVIDIA Driver 195.36.24
and xorg.conf with "RegistryDwords" "EnableBrightnessControl=1"
So far with Kubuntu 10.04, I can configure the power management to set the brightness on logon.
And I can also change the brightness by writing directly to /proc/acpi/video/NVID/LCD/brightness
The package 'samsung-backlight' just enables the brightness to be controllable.
Instead, the problem you are experiencing is probably caused by a KDE bug
I have the same problem when pressing Fn-up & -down: current brightness level is shown, but it doesn't change. So I tried to install the samsung-backlight package and I observed the same result as someone else: when pressing up & down, the BL flickers and goes respectively to max & min.
So I rmmod-ed the BL module and after that, the behavior was normal! I could go through all steps, one by one. My next try was then to blacklist the module and reboot: but I was back to showing level without being able to change it. And modprobing the module didn't change that frozen behavior.
So it seems that loading the module at boot time does something special that can't be done afterwards. Maybe this can help you change something.