Defensibility is created by means of demand-side economies of scale: having lots of users, in other words. Think reddit, and digg, for instance. Easily cloned, but reasonably defensible due to the large numbers of users.
All of this still doesn't change the facts. I don't one reputable designer (and I know a lot of them) that will volunteer without knowing what they are working on.
To be honest, most reputable designers follow the no-spec rules (http://www.no-spec.com/) and wouldn't work on a project without some form of payment.
Your only offered form of payment is equity in the company - which right now is valued at $0 because no one knows what the company is about.
Um... You mean most reputable designers who have never been co-founders?
The guy is offering equity... Presumably a considerable amount (given that the project is in such an early stage). That is not remotely the same thing as spec-work.
That being said, people here are right-- there is virtually NO value on the table here, so the OP should be willing to give up some serious equity.
Better yet, the OP should add a designer-partner. Design is serious business and it's what separates the iPods from the Zunes... It's an ongoing process and should be baked into the way you make software... It certainly shouldn't be "slap a coat of paint on it and we'll make millions".
On the disclosure stuff, I agree. Stealth mode is 99% of the time ridiculous. That being said, there's no harm in being discreet. If I were in the poster's shoes, I'd post a vague description and give plenty of detail to any designer who wanted to learn more.
I think the difference comes down to "publish the idea on a web page where anyone can read it - including many people with a proven interest in startups, who are more able to implement it than the original poster" VS "disclose the idea on the phone to those who demonstrate some interest". Sure, leeches could certainly pose as the second type of person, but at least you've made a cursory attempt at weeding them out.
It is not that some one will start working on it without knowing what we are doing. We want to hear from hackers who are interested so that we can further discuss it.
I love YC, been to last Startup School, But hate to discuss our secrets openly here.