Comment 9 for bug 489041

So from that ACPI code it seems that giving you the brightness interface is rather the bug than it not working. There are multiple definitions for video devices (this is normal). One of them would probably show up under VGA (or PEGP), the other GFX0 is the one that seems to be present on your system. However the ACPI BIOS only defines functions to control brightness for the other device. Which makes it unlikely that using the functions on the other device has any effect on your real video device.

A dumb question in between: did brightness control ever work with Linux? If yes, which version? Somehow my guess would have been that acpi_backlight=vendor should allow the acer-wmi module to have a try on this. But did not work. Still, could you booth with that option and send me the dmesg from that? Also check whether there is something under /sys/class/backlight (thought I would suspect no).