There is plenty of documentation on www.ycombinator.com that answers this question.
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.