To share or not to share?

What's it with people claiming to be entrepreneurs who are tight lipped and believe everyone is out to steal their ideas? It makes no sense to me and I find it downright funny. If you won't share your idea because it is so easy to duplicate, you have a serious problem with your competitive advantage. It better not be something every Tom and Jane can replicate overnight! If it is something so cool that no one else can do it, what are you worried about? Think iPod. Think of the (lack of)success of its copycats.