People who think their startup's success is going to follow from their immensely valuable secret idea are disproportionately likely to have bad ideas.
--Paul Graham via news.ycombinator.com
Some level of secrecy can be good psychologically -- it's been shown that telling *everyone* about an idea that you *want* to do will cause reward centers in the brain to fire as if you had already done it.
For tech in general, and web startups in particular, unless you have some one-time-exploitable loophole that powers your business (unlikely) -- you're better off being a little more trusting than paranoid when talking to people who can help you. You have to give a little to get a little... or sometimes get a lot.