Unibody White MacBook display dims when laptop on top of another
I've had this happen to me a few times, and it is annoying. My best guess is that the magnets in one MacBook are tricking the other one into thinking that the lid is closed. I know that it's at least possible to intentionally trigger the magnet that detects if the lid is closed, and my guess is that the other MacBook has a magnet in close proximity that's triggering it.