Long-known to robotics researchers and cognitive psychologists, the problem of the Uncanny Valley is lately of most interest to games designers.
Basically, it's the puzzle that the closer we get to simulating human beings, we seem to retain a near-universal and infallable ability to distinguish between the fake and the real. It seems to be almost hard-wired into us. Wikipedia article here.
But now a company named Image Metrics has come closer than I've ever seen to jumping the chasm. Watch the video and be prepared for a surprise about the 1:20 mark.
Warning: Sound, but no screaming zombies.