When unplugging the ac adapter on my thinkpad X230(x86_64 Gentoo Linux) the screen doesn't get dimmed, even though the icon is changing. This feature works on my older thinkpad X61(x86 Gentoo Linux), which is why I realized it.

power-manager with --debug shows that the change in power supply is realized.
The thinkpad X230 has a "Intel Corporation 3rd Gen Core processor Graphics Controller" (lspci), which can be controlled via the acpi_video0 in /sys/class/backlight, but it also works via the intel_backlight interface (adding this to backlight_interfaces does not help.) Also the brightness plugin on the panel works and the fn keys work when xfce's volumed is running.