Comment 6 for bug 741223

I'm having about the same problem in my Acer Aspire 5742Z. Hotkeys bring up the brightness window, and they modify the values of the files in the /sys/devices/virtual/backlight/acpi_video0 interface, but the actual brightness isn't really changed.

A minor difference with the original reporter is that any changes I make in the acpi_video0/brightness file don't work after reboot either.

Booting with "acpi=off" makes my brightness keys actually work again (am I losing anything more significant than brightness with "acpi=off"?).
On the other hand, "acpi_backlight=vendor" or "acpi.brightness_switch_enabled=0" don't have any effect, brightness doesn't work with them.

I upgraded by BIOS to the latest available firmware, and I'm attaching the DSDT.dsl output as suggested in https://wiki.ubuntu.com/BIOSandUbuntu#Buggy%20DSDT