Thanks for your answer.
I have tested 2.6.29-1 kernel: it's a better but there are still 2 issues:

First, there are 2 keys with an input event >255 (0x0ff):
-"hotkey ATKD 0000005c 00000000" mapped to BTN_EXTRA (0x179). I think it should be mapped to KEY_SCREENLOCK instead as this is what this button is supposed to do.
-"hotkey ATKD 0000008a 00000000" mapped to KEY_TV (0x114). I don't know what we could use instead.

The second problem is that for certain buttons, events are reported by 2 input device (so event triggered 2 times). It affects keys mapped to:
KEY_PLAYPAUSE
KEY_NEXTSONG
KEY_PREVIOUSSONG
KEY_STOPCD
KEY_VOLUMEDOWN
KEY_VOLUMEUP
KEY_MIN_INTERESTING
The two input devices that report events are these ones (from "hal-find-by-capability --capability input | xargs -n1 lshal -u"):

udi = '/org/freedesktop/Hal/devices/computer_logicaldev_input_3'
  info.addons.singleton = {'hald-addon-input'} (string list)
  info.callouts.add = {'debian-setup-keyboard'} (string list)
  info.capabilities = {'input', 'input.keys', 'button'} (string list)
  info.category = 'input'  (string)
  info.parent = '/org/freedesktop/Hal/devices/computer'  (string)
  info.product = 'Asus Laptop extra buttons'  (string)
  info.subsystem = 'input'  (string)
  info.udi = '/org/freedesktop/Hal/devices/computer_logicaldev_input_3'  (string)
  input.device = '/dev/input/event8'  (string)
  input.product = 'Asus Laptop extra buttons'  (string)
  input.x11_driver = 'evdev'  (string)
  input.xkb.layout = 'fr'  (string)
  input.xkb.model = 'pc105'  (string)
  input.xkb.rules = 'evdev'  (string)
  input.xkb.variant = 'oss'  (string)
  linux.device_file = '/dev/input/event8'  (string)
  linux.hotplug_type = 2  (0x2)  (int)
  linux.subsystem = 'input'  (string)
  linux.sysfs_path = '/sys/devices/virtual/input/input8/event8'  (string)

udi = '/org/freedesktop/Hal/devices/platform_i8042_i8042_KBD_port_logicaldev_input'
  info.addons.singleton = {'hald-addon-input'} (string list)
  info.callouts.add = {'debian-setup-keyboard'} (string list)
  info.capabilities = {'input', 'input.keyboard', 'input.keypad', 'input.keys', 'button'} (string list)
  info.category = 'input'  (string)
  info.parent = '/org/freedesktop/Hal/devices/platform_i8042_i8042_KBD_port'  (string)
  info.product = 'AT Translated Set 2 keyboard'  (string)
  info.subsystem = 'input'  (string)
  info.udi = '/org/freedesktop/Hal/devices/platform_i8042_i8042_KBD_port_logicaldev_input'  (string)
  input.device = '/dev/input/event4'  (string)
  input.originating_device = '/org/freedesktop/Hal/devices/platform_i8042_i8042_KBD_port'  (string)
  input.product = 'AT Translated Set 2 keyboard'  (string)
  input.x11_driver = 'evdev'  (string)
  input.xkb.layout = 'fr'  (string)
  input.xkb.model = 'pc105'  (string)
  input.xkb.rules = 'evdev'  (string)
  input.xkb.variant = 'oss'  (string)
  linux.device_file = '/dev/input/event4'  (string)
  linux.hotplug_type = 2  (0x2)  (int)
  linux.subsystem = 'input'  (string)
  linux.sysfs_path = '/sys/devices/platform/i8042/serio0/input/input4/event4'  (string)


So it seems like /dev/input/event4 should not report them?

I can provide more infos if needed, or make any test.
I can understand if it's not possible to make the backport. But I would really like to have it working for karmic.
Anyway, is there some workaround for 2.6.28?
Thanks