Even in literature and art, no man who bothers about originality will ever be original: whereas if you simply try to tell the truth (without caring twopence how often it has been told before) you will, nine times out of ten, become original without ever having noticed it.
C. S. Lewis
Probably one of my biggest pet peeves is a beginning writer telling me they're afraid to join a critique group or even send out their work because someone could steal their unpublished ideas or words (I guess they think it's that brilliant). These are the same people who mail themselves a copy, write copyright and the year on every page and are generally paranoid. This includes almost every writing conference I've attended where someone gets up and rails about their work being stolen by some large entity, a major magazine, a film studio, Steven Spielberg. Please. I can't recall the quote but every story has been told before, it's your unique slant that makes it original work.
This is not to say your published work can't be stolen, or used on another website as content. A good article that addresses website piracy. And yes some authors have had chunks of their work stolen completely, remember Opal Mehta?
Does it happen, yes, should you be obsessed that it could happen to you? I don't think so. Now get back to work!
The Writing Nag