You Should Write What You Know
I think this depends on how you define the word know. Do you have to be an expert on a subject to write about it? I hope not, because I enjoy writing about many things which are completely unfamiliar to me, zombie rabbits for instance. But I do know animals. I volunteer as a docent at the zoo and have come to appreciate animals in a whole knew way.
On the other hand, when people know too much about their subject it can stifle the action and the tension of a scene. I’ve read several manuscripts (none of them published) where the writer spent so much time trying to teach and inform us about his favorite topic, that the story was drowned.
Lynda Young did a great post about this on her site a couple months ago. http://lyndaryoung.blogspot.com/2010/10/write-what-you-dont-know-part-1.html
Do you only write what you know, or do you like to explore the unfamiliar?