Why not put the idea out there and ask for help in building it?
I've got to remind myself to not be so paranoid. There aren't people spending their whole days lurking to steal 'your' idea. Even if they did, the vision isn't there to see it through.
Really it's worth far more to get a wide variety of opinion and critique, and potentially a cofounder, than it is to guard the idea for fear of having it stolen.
Here: http://bit.ly/Z8U5U