I'd say there is little value in keeping
any idea to yourself. Ideas are incredibly cheap[1]. First mover advantage has also been shown not to be very important - just look at Facebook vs earlier social networks as an extreme example. Its all about execution - anyone can come up with good ideas (hell, YC is now even experimenting with accepting teams
without ideas). Besides, lots of people have ideas all the time and very few people go through with them. The going through with them is the valuable part, not the having ideas part.
[1] I was at Startup Weekend recently and our team (5 of us who had never met before the Friday night) turned a vague and impossible concept into a workable product prototype including business plan, market validation and pitch to investors in ~40 hours. We started out because we felt it would be a fun project to build crazy multiplayer game AI and ended up with an analytics solution that real game developers told us they would happily pay for to being approached by an investor asking us to continue working on it - all in the space of a weekend of focused brainstorming. If a focused weekend is all it takes to come up with a refined idea, basic prototype, business plan, market validation and pitch to investors, then I don't think its worth worrying about if someone will steal your idea. Most startups fail anyway and I bet very very few, if any at all, failures are because the company didn't keep quiet about their idea.
PS: the one time when it may make sense to keep your ideas secret is if you are told to, eg, by an investor who's about to give you a lot of money (but be careful! don't want the investor to back out after you've told everyone else that you can't talk about your business...). Outside of that special case, I think you need to give your idea away at some stage, eg, at a minimum for market validation or to tell investors about it (they won't sign NDA's).