If you’re having the problem on a T400 where you can’t change screen brightness, even after installing all the correct drivers, try the following:

Right click on the green battery on your taskbar, go to ‘Switchable Graphics’ and select ‘Energy Saving.’

You should now be able to change your screen brightness both via the function keys, and automatically via your power profiles.

Update: Turns out this is a bug with the original Bios which was shipped with the T400. Jump on to the Lenovo website and download the updated bios to fix it all up.