[jsilence]

Not sure about this one. Fiddling a little bit, I'd rather say that there is a light sensor left of the camera. Blanking only the camera does nothing, blanking the spot just left of it triggers the backlight dimming.

[Argorak]

To be precise: in old macbook pro versions, the sensor was in the speakers. The unibody macbook moved it next to the camera:

http://osxdaily.com/2010/05/22/where-is-the-ambient-light-se...

[meepmorp]

Yeah, this used to cause trouble for me at work. The way my office was lit, moving my hands too far over the (right?) speaker would suddenly change the kb lights and screen brightness. Turned the auto brightness after a couple hours and never re-enabled it.