Hi Karate (just had to say that),
Don't worry about it. First, because your idea will be miles ahead of anyone who hears it, there's no point in trying to steal it. You'll finish far before anyone else could. Second, you have thirty or so witnesses that heard you pitch the idea. If someone were to try, you'd have a good case against them.
What is more likely is that someone might hear your work, and think, I would have gone an entirely different way with that concept. They may write a totally different story based on your initial premise. In that way, you would have been their muse, but the works, yours and hers, would be separate stories.
Fear not, Board-breaker, attend your workshop, make friends and connections, hone your craft, and have a good time.
Craven