I've dealt with a few clients like this. Most wouldn't say anything till I signed an NDA, usually the first day of working on the project. However, with some, if I came out and said I wasn't comfortable until I knew more, and was willing to sign an NDA, they'd agree, I'd sign, and get the whole spiel.
Here comes the bite: Some would respond to the request by showing me the door.
Information is paramount in business, and some people are simply unwilling to risk letting anything leak till they're ready.
I absolutely agree that faith in a project determines how much effort beyond the norm I put in. [0] However, sometimes you'll just get a job that needs the professional treatment, because it will fail no matter how well you do: but its still a paycheck, and you still have a contract.
If they end up unwilling to let you know more, then try and judge the product on its caretakers, the people you meet. It might be worth the risk of a terrible tedious failure. [1]
[0] On a bad project, I work my hours. On something I believe in, I've been found sketching relationship trees at 4AM because I had an idea.
[1] The last project I had was terrible. The inception idea was fantastic. It got gutted twice over whilst I was still building it. I went from senior developer, to junior. I lost creative input. So, I was pissed off, exhausted with politics, and demoralised. However, I still got the project in on time, to specification, and got a nice paycheck.
Edit: Why I did [1].