Researchers have studied the curious effect humanoid robots have on the human brain which is isn't sure how to process a mechanical "person." The closer the robot is to looking human the more uncanny it is, however robots that don't resemble people are fine. So basically they have come to the conclusion that robots are creepy.
The actual "valley" refers to a precipitous drop in "likeability" as onscreen characters and humanoid robots step too far towards being human-like. As in, we enjoy Pixar's Wall-E and Nintendo's Mario, but we get the heeby jeebies from the ultra-realistic faces of The Polar Express or the upcoming Tintin movie.
So far, the phenomenon has been described entirely anecdotally, but an international team of researchers, led by Ayse Pinar Saygin of the University of California, San Diego, wanted to find out if the sensation was actually caused by something deep within our brains.