It seems something else than
gnome-power-manager manages the backlight dim timeout on Thinkpads, maybe
How do you alter the timeout from 5 seconds? (which is very annoying)
I had this issue on a Thinkpad x220. The following message showed up in /var/log/dmesg:
You can tell linux to let a vendor specific driver (thinkpad_acpi) handle the backlight by passing the acpi_backlight=vendor option to the kernel.
To do this:
The backlight is now managed by thinkpad_acpi and does not dim after 5 secs.