This is a nice post from over at Get Me Writing, and this is a point I very much agree with:

... if you’re showing people for feedback, and you’re worried about them stealing it, you don’t trust them. If you don’t trust them, you probably don’t trust their feedback either. So you shouldn’t show it to them. But don’t let that stop you from showing it to someone else that you do trust.

Honestly, if you don't want to show anyone your work because you think they'll pinch your awesome idea, then, well, you sound just like my mum. She was even hesitant when I told her I'd sent my novel out to my agent. Not any old agent. My agent.

So long as you know and trust the person you're sharing your work with, you'll be absolutely fine. Plus, I refuse to believe that the world is even slightly populated with cleptowriters. Most of us are lovely. Aren't we?