(From Ubuntu bug report:)
From HAL debug output when starting up:
: 14:13:54.913 [D] addon-macbookpro-backlight.c:535: Failed to detect ATI X1600, aborting...
My card is an NVidia...
On pressing a brightness-up or -down button:
14:14:15.824 [D] hald_dbus.c:3153: udi=/org/freedesktop/Hal/devices/usb_device_5ac_21b_noserial_if0_logicaldev_input
From gnome-power-manager --verbose --debug power:
[hal_device_condition_cb] gpm-button.c:391 (14:19:23): condition=ButtonPressed, details=brightness-up
[emit_button_pressed] gpm-button.c:335 (14:19:23): emitting button-pressed : brightness-up
[button_pressed_cb] gpm-manager.c:996 (14:19:23): Button press event type=brightness-up
[button_pressed_cb] gpm-srv-screensaver.c:167 (14:19:23): Button press event type=brightness-up
[button_pressed_cb] gpm-backlight.c:561 (14:19:23): Button press event type=brightness-up
[gpm_brightness_lcd_get_hw] gpm-brightness-lcd.c:108 (14:19:23): ERROR: Method "GetBrightness" with signature "" on interface "org.freedesktop.Hal.Device.LaptopPanel" doesn't exist
*** WARNING ***
[gpm_brightness_lcd_get_hw] gpm-brightness-lcd.c:113 (14:19:23): GetBrightness failed!
[gpm_brightness_lcd_set_hw] gpm-brightness-lcd.c:155 (14:19:23): Setting 11 of 228
[gpm_brightness_lcd_set_hw] gpm-brightness-lcd.c:165 (14:19:23): ERROR: Method "SetBrightness" with signature "i" on interface "org.freedesktop.Hal.Device.LaptopPanel" doesn't exist
*** WARNING ***
[gpm_brightness_lcd_set_hw] gpm-brightness-lcd.c:170 (14:19:23): SetBrightness failed!
[gpm_brightness_lcd_up] gpm-brightness-lcd.c:362 (14:19:23): emitting brightness-changed (0)
[brightness_changed_cb] gpm-backlight.c:736 (14:19:23): Need to display backlight feedback value 0
[gpm_feedback_display_value] gpm-feedback-widget.c:144 (14:19:23): Displaying 0.000000 on feedback widget
[gpm_refcount_add] gpm-refcount.c:100 (14:19:23): refcount now: 1
[gpm_refcount_add] gpm-refcount.c:101 (14:19:23): non zero, so sending REFCOUNT_ADDED
[brightness_changed_cb] gpm-backlight.c:740 (14:19:23): emitting brightness-changed : 0
[button_pressed_cb] gpm-srv-brightness-kbd.c:139 (14:19:23): Button press event type=brightness-up
[button_pressed_cb] gpm-info.c:698 (14:19:23): Button press event type=brightness-up
[gpm_refcount_auto_decrement] gpm-refcount.c:74 (14:19:25): zero, so sending REFCOUNT_ZERO
[gpm_feedback_close_window] gpm-feedback-widget.c:134 (14:19:25): Closing feedback widget
The pommed project  apparently had the same problem. They recently added support for nvidia cards , so maybe this could be ported to hal, too?