I don't talk about a story idea until I've completed a first draft. The closest thing is to ask someone a question about some detail, and find I'm forced to explain an aspect of the story for the person to be able to answer. And even that makes me horribly uncomfortable, and I've resolved to avoid doing it again at all costs. I have two reasons for this:
1. More often than not when I have done this, the person(s) I told my story to will find some flaw in it or come up with a helpful suggestion, which destroys the idea.
2. (And this is the main one) Talking about the idea relieves the pressure to write it and get it out of my head.
I have a friend who loves to hash out her stories over the phone and it's really hard for me to cooperate with that. Inside, I'm screaming, "Dear god, stop now before you kill this thing!" And, I have to say, she rarely completes a story.
How about you? Do you ever talk about an idea before you've written a first draft of it?