I agree with what most people are saying here, but I could add a few thoughts:
1.) Back in the early days of Fluther, I refused to talk to anyone about the idea. But it's hard to "hire" people for an idea you can't talk about. Now I'm convinced in general the risk of getting an idea stolen is quite low--it just takes a ton of time and commitment to start a company, and very few people will quit the job to try. So I wouldn't be too anxious about this, even though I know you are. What I would do is find a few trusted advisers in the industry who can weigh in on the idea and its merit, and help you decide how to proceed.
2.) Back in the day, I also filed a provisional patent on Fluther. This is cheap and will protect you for a year, but I agree with Sam. Don't bother. Software patents are a big mess, and very expensive. For a tiny startup, they just don't make sense. Users are more important. Friendster has the patent on social networking, but methinks they'd happily trade places with myspace or Facebook.
3.) Even if you can't find any similar sites, there are already people working on your idea (esp. if it's a simple/great one). I promise. As a matter of fact, they will probably beat you to market. Be prepared for that. That's doesn't mean you won't succeed. If you really believe in this idea, start on it now. Start on it yesterday. Seriously.
4.) I'd recommend reading Guy Kawasaki's "Art of the Start." It will get your blood pumping.