(I'm aware that there are differing theories out there on the merit of shrouding a startup idea in secrecy, and my own personal opinion on the matter is that it totally depends on the idea. But the question still holds.)
Incubators like ycombinator have seen thousands of start-up ideas over the years. Most likely some of them overlapped, and there has probably been a good amount of duplication (deal of the day site--only for food!). If there was an NDA for every single one of these, any successful company that might have duplicated an idea unintentionally (ie, they came to the same idea themselves) might get in trouble later down the road when another entrepreneur says "hey, that was my idea, they must have stolen it!"
EDIT: seems like me and iqster had the same basic comment :)
Ideas are worth less than a dime a dozen. Being successful comes down to execution. In my experience, if an idea is really good, a number of people will come up with it at around the same time.
No, we won't sign an NDA. No venture firm would at this stage. The informal commitment to secrecy on our application form is more than any VC would make.
In this connection you may want to read the first section of How to Start a Startup on the value of mere ideas."
Thanks, I missed this. The "informal commitment" is what I was looking for.