Ramit Sethi has a great blog on personal finance - IWillTeachYouToBeRich.com - and I've posted comment which I'll share with TIG here:

In response to Your idea isn't good enough to keep secret:

"Wholeheartedly agree with this post. Meeting someone who is excited about their idea is energizing, and keeping your idea secret can sap the energy that you'd otherwise exude. Even when it is obvious that a young entrepreneur is inexperienced and has a lot to learn, the expression of their energy and drive can go a long way toward gaining the trust and backing of people (mentors, future colleagues/customers) who can help them get where they want to go.

Clear vision and strong motivation are rare enough commodities that showing them off by telling others about your idea are more likely to align people behind you than inspire competitors to stop you. The world is big. Business 'ideas' are everywhere. Never underestimate the value of a person who can take an idea and make it happen -- by opening their mouth they are far more likely to get hired or approached by a possible client than they are to spark a competitive conspiracy against them.

Of course there are exceptions -- I'm assuming that we've all got some judgment and will learn-as-we-go what is appropriate in specific circumstances. I just hate to see young people (like myself) pass up opportunities that are out there because we are too eager to believe that ideas are so scarce that we must remain quiet about our own.

When your passion shows through the way you present your idea, and you exude a sense of openness and willingness to accept feedback in order to take your idea and make it happen, I think that people would be more inclined to ask how they can get behind you than how they can take this away from you."