I suffer from GAD (generalized anxiety disorder) and I think that SA is just a part of GAD, for me at least. Thumbs-up for LaRibbons post. Now I'm looking through Wikipedia for mental disorders, haha. It's kinda nice to realize that there are far worse things out there than SA - kinda makes you appreciate things a bit more.
I have worked with autistic children for the last four years or so, and a large component of autism is anxiety around people and social situations. It goes much deeper and affects a person's life in different ways, but social anxiety is a large component in autism. And, the autistic population is growing rapidly, so we may see more autistic people in this forum (I've noted at least one so far). Aspergers, mentioned earlier, is another one that is on the autistic spectrum and has a large social anxiety component.