Fascinating article in the November 2006 issue of Wired magazine about Prosopagnosia or face blindness.
Prosopagnosics cannot recognize faces; they are otherwise usually quite functional and can recognize people using auditory or contextual cues. They sometimes don't even recognize themselves in the mirror. The article does a nice job of explaining the problem from the perspective of several different sufferers, as well as a biography of a researcher who is very interested in the topic.
Neuroscientist Brad Duchaine wanted to work with prosopagnosics but he thought they would be hard to find; it was previously believed that the disorder only occurred in stroke victims or others who had been shot in the brain. Luckily, he was referred to Bill Choisser who had started a Yahoo! Group for other folks suffering from this disorder. (It started as a Usenet group that has since morphed into a Yahoo! group). Now that Duchaine had some sufferers to interview and study, he was able to conduct some research. He works the Prosopagnosia Research Centers at Harvard University and University College London; see his list of publications (many in pdf).
What was most helpful to me was the photographs which gave a sense of what face blindness must be like to those 2% of Americans (or 6 million?!) who suffer from prosopagnosia. Here's another image (from Choisser’s book Faceblind) that illustrates the problem.
You can read more about prosopagnosia from the Wikipedia or browse the material at the Prosopagnosia Research Centers’ research page .