This worked for my X61:
Once your brightness keys work, you can set the brightness by writing to the procfs:
# echo 100 > /proc/acpi/video/VID1/LCD0/brightness
using a script and acpid events. However this may lead to a bug (screen flickers) described on the Debian mailing list []. This bug is related to the X server as it does not occur on the tty1. To fix it change your xrandr backlight control to native:
$ $xrandr --output LVDS --set BACKLIGHT_CONTROL native
% cat /etc/acpi/video_brightnessdown.sh #!/bin/sh xbacklight -dec 10%
% cat /etc/acpi/video_brightnessup.sh #!/bin/sh xbacklight -inc 10%
If you are using a 2.6.26 kernel, you are supposed to use the regular ACPI backlight control instead of thinkpad-acpi, on any Lenovo (Windows Vista-capable) ThinkPad.
Modify the kernel config by first disabling CONFIG_THINKPAD_ACPI_VIDEO and then enabling both CONFIG_VIDEO_OUTPUT_CONTROL and CONFIG_ACPI_VIDEO  While you are at it, be sure to read the well-written help sections for the kernel config settings :)
If the following commands fail, you may want to try other module parameters for thinkpad-acpi. I had to use backlight_mode=1 for example on my T20. Just add the following line to your /etc/modprobe.conf
options thinkpad-acpi brightness_mode=1
See the chapter LCD brightness control of Documentation/laptops/thinkpad-acpi.txt in the kernel sources for more informations.
To adjust the brightness to a certain level:
# echo 'level 3' > /proc/acpi/ibm/brightness
Just one level up or down:
# echo 'up' > /proc/acpi/ibm/brightness
# echo 'down' > /proc/acpi/ibm/brightness
See /proc/acpi/ibm/brightness for a list of all available commands.
The sysfs-interface allows more a flexible method of brightness control:
Get the current brightness level:
# cat /sys/class/backlight/thinkpad_screen/actual_brightness
Get the highest brightness level:
# cat /sys/class/backlight/thinkpad_screen/max_brightness
Adjust the brightness to a certain level:
# echo 3 > /sys/class/backlight/thinkpad_screen/brightness
Ubuntu 8.10 Intrepid Ibex
The hardware keys work out of the box on a clean install of Ubuntu 8.10 Intrepid Ibex. No tweaking necessary.