Comment 33 for bug 826386

I found a temporary fix that works very well. It is not perfect, but it is far better, and far more convenient, than the "setpci" fix. With this fix, the backlight does not automatically come on, but it makes the brightness control keys functional again, and will turn on the backlight with just a keystroke. This fix works with my Acer Aspire 5734z. Full credit goes to Stryker777 of the Ubuntu Forums. He proposed this fix during Natty Narwhal Alpha 2. The following is his post, taken from the Ubuntu Forums Archive.

  "I had to hit shift to get into grub, select a previous version to boot, then once booted, edit /etc/default/grub with the following:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
GRUB_CMDLINE_LINUX="acpi_osi=Linux"

then run:
sudo update-grub

Now I can hit my brightness buttons to get the backlight to come on.

Temp fix but works on my Acer Aspire 5734z"

This fix works with Oneiric Ocelot. You can use "sudo setpci -s 00:02.0 F4.B=0" to activate the backlight, then make changes to grub. Reboot for the change to take effect. As I have stated, the backlight will NOT turn on automatically, but brightness control keys are functional, and will turn on the backlight. This even works resuming from "suspend" and "hibernate".