The New York Times has just published an article on dissociative fugue, the poorly understood memory disorder where people seem to forget who they are.
It has many similarities to conversion disorder where people seem to experience a disability (such as paralysis) despite having nothing medically wrong with them.
Both conversion disorder and dissociative fugue are often linked to trauma and they are often thought to arise from emotional difficulties being pushed from consciousness and expressed in other ways – all outside the conscious control of the patient.
Brain imaging research has shown that these sorts of states are likely to be different from people purely ‘faking’ the same thing, but in the clinic, fakers might be still quite hard to detect and extensive neuropsychological testing may be required to do so.
Also, there’s always the worry that there is some medical reason for the problem that just hasn’t been found yet.
Despite these difficulties, some researchers are investigating these conditions, which may provide vital clues to understanding the conscious mind.
The article discusses some famous cases of dissociative fugue and deal with some of the differences with amnesia after brain injury.
It also mentions that a play about the condition, entitled Fugue, is running at New York’s Cherry Lane Theatre until April 22.
Link to NYT article ‘When a Brain Forgets Where Memory Is’.
Link to description of dissociative fugue from the Merck Manual.
Mind Hacks 
Science of Insanity (url above) has an interesting first-person account of a dissociative fugue — better than the gee-whiz NYT story.
The URL referred to by MishaD above is http://scienceofinsanity.blogspot.com/2007/03/what-is-it-like-to-be-in-dissociative.html