Thanks so much for the help, Toz! The laptop is a g7-1310us.
~ $ cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-3.2.0-23-generic root=UUID=c254b4b8-5b7e-4e53-9424-c513669fe3b5 ro quiet splash vt.handoff=7
When I change the brightness manually, the value in acpi_video0/brightness changes, so the line I have in rc.local is:
~ $ for i in /sys/class/backlight/*; do echo $i; cat $i/brightness; cat $i/max_brightness; cat $i/actual_brightness; done
As I mentioned, this worked for a little while.
echo 10 > /sys/class/backlight/acpi_video0/brightness
~ $ lspci -k | grep -A3 VGA
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09)
Subsystem: Hewlett-Packard Company Device 1671
Kernel driver in use: i915
Kernel modules: i915