This person has an intense and obsessive level of focus on things of interest and is often characterized by special (and possibly peculiar) gifts; one person might be obsessed with 1950s professional wrestling, another with national anthems of African dictatorships, another with building models out of matchsticks. Particularly common interests are means of transport (for example trains), computers, and dinosaurs. These interests are often coupled with an unusually high capacity to retain and recall encyclopedic amounts of information about the favored subject. In general, orderly things have appeal to these individuals, and they often manifests extremely sophisticated reason, an almost obsessive focus, and eidetic memory.
Sound like most of the developers you've ever met? Well, it's the Wikipedia definition of Asperger's Syndrome, a really mild form of Autism. Instead of seeing something like autism vs "normal" mental development, it's really more of a sliding scale. You've often heard that developers have special in-born characteristics -- I think these folks are a little more towards Aspergers than most. This explains a lot about the social skills and concentration abilities of really brilliant developers who don't interact well with the other humanoids. And why we can't get dates.
Update: Coincidentally, the very night that I posted this blog, I went to dinner with someone (very well respected writer and speaker) who has a child with Aspergers. He was telling me that this is a real problem in Silicon Valley -- geeks marrying geeks and having borderline autistic children. Talking to him made me realize that this entry might be construed to trivialize Asperger's Syndrome. That is not my intent at all -- it is a serious condition. However, talking to my friend drove home the point I made above even more strongly.