When I meet someone who won't tell me what their idea is, usually I've thought they were a little silly--as if they thought I was a spy who would sell their idea on some sort of idea black market. Generally, ideas are not viral--you'll tell someone, and then, frankly, it's not interesting enough for them to pass it on.

There is a place for secret sauce, it's just that the existence of the startup and it's business model is not likely to be it. Few attacks succeed as brilliantly as a surprise attack. If some part of your idea is novel, but fairly easy replicated, for example the Air's touchpad, I would keep that under wraps to extend your advantage.