From the application form:<p>"We don't make any formal promise about secrecy, but we don't plan to let anyone outside Y Combinator see these applications, including other startups we fund."
They're going to give your ideas to your closest competitors. Haven't you heard of Competitor Day?<p>/old<p>But seriously, ideas are totally worthless. I thought my totally awesome idea for a productivity site was worth a billion dollars too. And I built it and released it, and you know what? Nobody cared, and rightly so, because it was poorly executed. Now I'm playing catch-up with the design by making things cleaner, streamlined, and simple. <i>That</i> is what matters, not whatever your idea is. Though it's not until you've been steamrolled by brett and his clickable calendar that one can really understand that, maybe. :)
To play devil's advocate, what if YC folks have already thought of the idea you had(and obviously did not pick you for reasons other than your idea)? See, it goes both ways. <p>Bottomline: it is very difficult in this environment with start-ups launching left and right to claim ownership of a certain idea, even between two parties.