Controlling the ThinkLight with the keyboard works on all systems since it relies on the BIOS exclusively. Just press Fn-PageUp to toggle its state between on and off.
Support for controlling the light with ACPI is provided by ibm-acpi. After installing it, a simple
# echo on > /proc/acpi/ibm/light
switches it on and a
# echo off > /proc/acpi/ibm/light
switches it off again.
This allows one to control the light in scripts. Unfortunately, no known ThinkPad comes with a light sensor (yet). ;)