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.
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 love YC, been to last Startup School, But hate to discuss our secrets openly here.