Here's what HP had to say:
Some of you may have seen or heard of a YouTube video in which the facial-tracking software didn’t work for a customer. We thank Desi, and the people who have seen and commented on his video, for bringing this subject to our attention.Via MightyGodking.
We are working with our partners to learn more. The technology we use is built on standard algorithms that measure the difference in intensity of contrast between the eyes and the upper cheek and nose. We believe that the camera might have difficulty “seeing” contrast in conditions where there is insufficient foreground lighting.