In my case, I'm working nights/weekends --- solo --- on a web application. What scares me is if I tell too many people what I'm doing, someone who can put it together faster than I can (smarter, more people, more time, whatever) and leave me in the dust will obviate my work. I don't consider that stealing. I just don't want to get beat. My idea isn't even that novel or anything - it just isn't available as I envision it.
It's a frustrating thing, too, because I think NDAs at this stage are just silly, and I need to find at least one other person to work on this with me (preferably local to DFW). But keeping my hand close to my chest is the natural defensive reaction. It's hard to get over.