I have now two different devices that refer to the same hardware:

lrwxrwxrwx 1 root root 0 2008-09-06 11:04 acpi_video0 -> ../../devices/virtual/backlight/acpi_video0/
lrwxrwxrwx 1 root root 0 2008-09-06 11:04 toshiba -> ../../devices/virtual/backlight/toshiba/

Unfortunately, due to ACPI implementation the acpi_video0 one is much
inferior (as it provides only effectively two levels instead of 8);
and user level tools are apparently quite confused which one to select.

Is there any mechanism that would allow tochiba_acpi to claim brightness
for internal LCD screen that video would not attempt to grab it too?

Of course manually disabling brightness handling in video is always possible,
still is nice for this to be handled automatically.