Assuming that your post is representative of a genuine concern for people with real problems and not just YAILOT (yet-another-ignorant-lack-of-tolerance), the DSM IV term you are looking for is OCD, “obsessive-compulsive disorder.” According to this site,
There are further qualifications on that site that must be present to indicate that the disorder exists. As they note, however, no diagnosis can be made except by a psychiatrist.
OCD-type obsessions are not really the same as ‘being obsessed about Star Trek’. OCD-type obsessive thoughts are more like intrusive ideas that are triggered by something unrelated to the idea itself. For example, a person with OCD might recall, in detail, a scene from an episode of Star Trek when crossing a certain street. OCD isn’t really all about this, but it’s one possible symptom.
Most ‘obsessions’ – whether they’re about Star Trek or work or science – are not pathological and are not obsessive-compulsive disorder. However, some people with certain conditions might be very likely to become obsessed about something.
Some of the geekiness and obsessiveness to one particular subject sounds a bit like Asperger Syndrome to me – especially the stuff in section B.
from the DSM:
No, I am not saying that loves D&D = Aspergers, but the few folks I’ve known who were REALLY into the sci-fi/fantasy thing fit much of this description. (FWIW, I used to be pretty immersed in the D&D thing, but I got better.)