gnome-power-manager fails to control LCD brightness, while FN keys work normally (Toshiba satellite A210P)

Binary package hint: gnome-power-manager

Toshiba satellite A210P with ATI graphic card, running Ubuntu 10.04 LTS kernel 2.6.31-10-rt (kernel 2.6.32 also affected).
issue in gnome-power-manager 2.30.0-0ubuntu1
After installing omnibook module (for bluetooth) and proprietary ATI driver gnome-power-manager cannot read and change LCD brightness. The applet slider is stuck at the upper position. Brightness can only be changed by pressing Fn keys. In karmic, everything worked fine.

The keys can be disabled by modprobe -r video. However, the situation with g-p-m remains.
Blacklisting omnibook did not solve the issue.

cat /proc/acpi/video/VGA/LCD/brightness
returns:
levels: 0 10 20 30 40 50 60 70
current: 70

Trying to echo anything to /proc/acpi/video/VGA/LCD/brightness fails with access denied error, even as superuser.

Dmesg after pressing Fn keys returns:

[ 1802.062534] [fglrx:firegl_acpi_video_event] *ERROR* Could not find private acpi context by video busid: LCD
[ 1813.759336] [fglrx:firegl_acpi_video_event] *ERROR* Could not find private acpi context by video busid: LCD
[ 1813.773890] [fglrx:firegl_acpi_video_event] *ERROR* Could not find private acpi context by video busid: LCD
[ 1813.783447] [fglrx:firegl_acpi_video_event] *ERROR* Could not find private acpi context by video busid: LCD
[ 1814.933052] __ratelimit: 14 callbacks suppressed
[ 1814.933063] ACPI: EC: missing confirmations, switch off interrupt mode.
[ 1814.939167] [fglrx:firegl_acpi_video_event] *ERROR* Could not find private acpi context by video busid: LCD
[ 1814.956165] [fglrx:firegl_acpi_video_event] *ERROR* Could not find private acpi context by video busid: LCD

Adding GRUB_CMDLINE_LINUX=”acpi_backlight=vendor” disables both keys and g-p-m.

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: gnome-power-manager 2.30.0-0ubuntu1
ProcVersionSignature: Ubuntu 2.6.31-10.153-rt
Uname: Linux 2.6.31-10-rt i686
NonfreeKernelModules: fglrx
Architecture: i386
Date: Tue Jun 8 23:07:56 2010
GnomeSessionIdleInhibited: No
GnomeSessionInhibitors: None
GnomeSessionSuspendInhibited: No
InstallationMedia: Ubuntu 10.04 LTS "Lucid Lynx" - Release i386 (20100429)
MachineType: TOSHIBA Satellite A210
PccardctlIdent:
 Socket 0:
   no product info available
PccardctlStatus:
 Socket 0:
   no card
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.31-10-rt root=UUID=02cd411d-dee3-45c7-a5a8-7ddb5b5facaa ro quiet splash
ProcEnviron:
 LANG=ru_RU.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-power-manager
dmi.bios.date: 06/10/2008
dmi.bios.vendor: TOSHIBA
dmi.bios.version: V2.00
dmi.board.name: IALAA
dmi.board.vendor: TOSHIBA
dmi.board.version: 1.00
dmi.chassis.asset.tag: *
dmi.chassis.type: 10
dmi.chassis.vendor: TOSHIBA
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnTOSHIBA:bvrV2.00:bd06/10/2008:svnTOSHIBA:pnSatelliteA210:pvrPSAELE-03R00FRU:rvnTOSHIBA:rnIALAA:rvr1.00:cvnTOSHIBA:ct10:cvrN/A:
dmi.product.name: Satellite A210
dmi.product.version: PSAELE-03R00FRU
dmi.sys.vendor: TOSHIBA