I have a clean install of Arch with Gnome 3.8 and backlight control is not working which has worked previously OOB on all versions of Gnome and Arch. Kernel parameters from ThinkPad T420 article didn't work. I have acpid and tp_smapi installed and enabled whereas tp_smapi module loads automatically. I have a Thinkpad T420 with Fn+Home/End to control brightness. I am able to control brightness using gnome menu. All other function keys seems to work fine.
ls /sys/class/backlight/ intel_backlight thinkpad_screen
Try using these for your kernel parameter instead:
video.brightness_switch_enabled=0 acpi_osi='!Windows 2012'