As mentioned a few comments below, affect/emotion detection can be useful for people with autism. Rana El Kaliouby has been working on a system that does exactly this in the Affective Computing group at the Media Lab. My understanding is that they have built it into an appliance which is robust to poor lighting situations, which is pretty impressive. There is also a great deal of interest in this in the human-robot interaction and human-computer interaction communities in general, so that robots and computer systems can respond more naturally to people in social situations.