Comment 235 for bug 568611

[Oneiric Beta users]:

In Oneiric, the behavior "screen brightness keys toggle from max to min-brightness (no steps between)" in the Unity desktop is caused by bug 827517, which is now fixed in the gnome-settings-daemon package in the main Ubuntu archives -- but the fix didn't arrive until just after the Oneiric Beta 1 release. Simply updating your Oneiric system software after installing Beta 1 should pick up the fixed gnome-settings-daemon (version 3.1.90-0ubuntu2 or higher).

As noted previously, some Oneiric systems will still need to boot with "acpi_backlight=vendor" or some other vendor-specific boot parameter to make the brightness up/down keys work at all. See the PPA instructions page for details and additional diagnostic information (even though the PPA is no longer needed for Oneiric systems):
  https://launchpad.net/~kamalmostafa/+archive/linux-kamal-mjgbacklight

[Natty and Maverick users]: No news to report.