Comment 2 for bug 453748

The behavior has changed: Pressing fn-brightness-up/down while gnome-power-manager is running still causes actual_brightness to jump up/down two levels on each keypress, but it now represents the correct brightness (i.e. my display backlight really increases or decreases its brightness by two levels).
I believe that the keypress is both handled by my laptop directly and by gnome-power-manager. BIOS (or hardware directly, however this is actually implemented) reacts to fn-brightness-up by increasing the brightness one level. g-p-m also reacts to the keypress by increasing the brightness by one level. This causes the backlight to actually increase its brightness twice on a single keypress.

lshal reports for my backlight:

udi = '/org/freedesktop/Hal/devices/computer_backlight'
  info.addons = {'hald-addon-generic-backlight'} (string list)
  info.capabilities = {'laptop_panel'} (string list)
  info.category = 'laptop_panel' (string)
  info.interfaces = {'org.freedesktop.Hal.Device.LaptopPanel'} (string list)
  info.parent = '/org/freedesktop/Hal/devices/computer' (string)
  info.product = 'Generic Backlight Device' (string)
  info.subsystem = 'backlight' (string)
  info.udi = '/org/freedesktop/Hal/devices/computer_backlight' (string)
  laptop_panel.access_method = 'general' (string)
  laptop_panel.brightness_in_hardware = false (bool)
  laptop_panel.num_levels = 16 (0x10) (int)
  linux.hotplug_type = 2 (0x2) (int)
  linux.subsystem = 'backlight' (string)
  linux.sysfs_path = '/sys/devices/virtual/backlight/acpi_video0' (string)

laptop_panel.brightness_in_hardware being set to false is wrong, my laptop supports that.