The ThinkLight is a little LED light integrated into the top edge of the display frame in A, G, R, T and X series Thinkpads. Its purpose is to enlighten the Thinkpads keyboard so that you can type in the dark without using an external light source. The ThinkLight can be controlled via the Fn-PageUp key combination or via ACPI on more recent models.
Control via keyboard works on all systems since it's entirely done by the BIOS. Just press Fn-PageUp to toggle its state between on and off.
Control via ACPI is supported 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 way you can control it even by scripts. Unfortunately no known Thinkpad so far comes with a light sensor. ;)