I know you keep your brightness at different levels depending on what power source is currently in use. So maybe you could write two udev rules. One would have an action if /sys/class/power_supply/AC/online is a 1 and another if the file contains a 0.
The only problem with this is that you obviously would not be solving the original problem, but rather just working around it.
]]>Basically, when I boot, brightness is now at 100% (15). If I remove the AC, brightness is correctly set to 7. If I replace the AC, brightness is correctly set to 12. So this is hardly an insurmountable problem. However, it is annoying me that I can't figure out what is overriding my settings all of a sudden since it worked perfectly before.
Other packages upgraded at the same time include:
[2013-01-23 19:18] upgraded cairo (1.12.10-1 -> 1.12.10-2)
[2013-01-23 19:18] upgraded libglapi (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:18] upgraded libgl (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:18] upgraded intel-dri (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:18] upgraded khrplatform-devel (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:18] upgraded lib32-cairo (1.12.8-1 -> 1.12.10-1)
[2013-01-23 19:18] upgraded lib32-icu (50.1.1-1 -> 50.1.2-1)
[2013-01-23 19:18] upgraded lib32-harfbuzz (0.9.9-1 -> 0.9.9-2)
[2013-01-23 19:18] upgraded lib32-libxi (1.6.1-1 -> 1.6.2-1)
[2013-01-23 19:18] upgraded libgbm (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:18] upgraded libegl (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:18] upgraded libgles (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:20] upgraded linux-lts (3.0.59-1 -> 3.0.60-1)
[2013-01-23 19:20] upgraded mesa (9.0.1-1 -> 9.0.2-1)
[2013-01-23 19:20] upgraded refind-efi (0.6.4-1 -> 0.6.5-1)
but it might have been something upgraded shortly before this if it wasn't something which would have made me reboot. I'm wondering about intel-dri or the kernel but I'm not sure what the others do and, as I say, it could be something else.
I use KDE but in system settings, KDE is set not to automatically change the brightness for AC/battery/low battery although KDE is meant to dim the screen after a period of inactivity.
$ systemctl status laptop-mode.service
laptop-mode.service - Laptop Mode Tools
Loaded: loaded (/usr/lib/systemd/system/laptop-mode.service; enabled)
Active: active (exited) since Sul 2013-01-27 02:53:20 GMT; 24min ago
Process: 1078 ExecStart=/usr/sbin/laptop_mode init auto (code=exited, status=0/SUCCESS)