You know what, I have actually noticed something like that. I was typing fast and noticed that the keyboard area was rapidly changing in brightness - subtle but there. It's happened a few times, never been able to pin it down or reproduce it---OH that is weird, it literally started doing it the second I typed "reproduce". And now it's stopped again.
I have noticed the ambient light sensor is VERY sensitive - that's all I can suggest in the way of explanation.