I would have to think that unless your idea is something rather simple and quickly profitable (which is unlikely in most cases...), I would doubt anyone would really devote himself to it and make it happen. It takes a certain passion for an idea to actually become something.
There are millions of great business ideas. I really doubt your friend will ever do anything with the idea. I`m sure there are examples of ideas being stolen, but not enough to be paranoid about it. You`ll likely just scare off others who will be passionate about your idea with NDAs and junk like that.....
I say do as much work on the idea as you can yourself first. You likely have a vision of how it all will work. Read this book: Getting Real. http://gettingreal.37signals.com/
It is a great book ABOUT good web development. (free to read online now) The most important point of the book is that the actual programming is the last thing you should be thinking about. Which means you could probably "develop" most of your web site without involving others. It would save time, money and help keep the design true to your vision.
Anyway, I`m rambling a bit and it probably isn`t exactly what you are asking, but best of luck with your endeavour.