I've a sony vaio with geforce 8400M GT, and I've no support for backlight, no laptop_panel in hal, no entry in /sys/class/backlight
I've reported the bug in hal, in devicekit, in kernel, by nvidia...
In fact, a lot of people are affected with this problem on laptops from hp, samsung, sony and mac.
-Hal is deprieciated
-devicekit won't fix some device specific issue
-nvidia is not supporting backlight, as every manufaturer is managing it differently
-kernel team is not reacting
I'm waiting already 2 years for a solution ! I hope to have it fixed for karmic, if not, we all are to wait 6 other month !
Martin Pitt (hal/devicekit developper) said he could make it work if xorg could fix it in xbacklight...
There is already the utility smartdimmer (or nvclock) that is able to set or get backlight, it could be possible to use that code...