It's quite conceivable that someone will steal your idea, if it's a good one. The question is the degree to which they can impact your ultimate execution of the idea--and that depends on a range of factors, from market dynamics to your own ingenuity.

Whenever I hear a common refrain in an industry that something doesn't happen, I know immediately that it absolutely does happen, but that acknowledging it doesn't fit with the existing culture for some reason.