Commit 7c0ea45be4f114d85ee35caeead8e1660699c46f registers ACPI backlight
even if _BQC method (query current backlight level) is missing. The effect is:

during initialization video.c:acpi_video_device_find_cap() calls backlight_update_status(). It tries to fetch actual level via acpi_video_get_brightness() - but as _BQC is missing it just returns current value
as stored in memory - i.e. zero, because it was never set. So effectively
backlight_update_status() reset brightness to minimal value == 0.

This happens on Toshiba Portege 4000. Verified by reverting commit on top of
rc8.

On a side note, it would be nice to fit backlight device into actual ACPI
device tree instead of /devices/virtual.