The people who are smart enough to steal your idea, in a way that would be competitive, probably have their own ideas or would be willing to work together.
Also, talk is cheap. And ideas are just talk.
Look at Google -- they tried to sell their algorithm so they could stay in school, but no one was interested, so they had to put up or shut up. If a bunch of people already thought your idea was a good idea, it'd effectively be a mature space.
I'd modify that slightly...
'people already thought your idea was a good (MONEY SPINNER) idea , it'd effectively be a mature space. ...'
New ideas maybe easy to come up with but translation into a product is not easy. If you can see a new idea implemented & copy it you piggy back on the innovation of others thinking & implementing.
Compare for instance Flickr to say Zoomer. The later was developed post flickr & probably quicker. The point being Flickr came up with the wonderful clean url design, api design and concepts such as 'favourites'. It also showed there was a market. Zoomer copied lots of Flickr ideas without having to think, tinker & hack.
Talking to laypeople helps you solidify and lets you practice pitching what it is that you're doing. When you're working at a company, you don't have to explain yourself very much. But when you say you're doing a startup, I've found that most people will demand more of an explanation. However, most people don't have the patience in everyday settings to listen to more than 2 or 3 minutes of it. This helps you whittle out the core of what it is that you're trying to build. Call it a functional spec cleaning, if you want.
Talking about your ideas with technical people might give you insight into different areas you're not as familiar with. The more that I read, the more I realized everything is inter-related. But it's hard to have that kind of breadth in this day and age of specialization--there's just way too much to know. Talking to technical people will often times help you make connections and leads to other technical fields that will help you get a feel for where else you can dig or if you're digging in the right place.
That said, most people won't steal your ideas for the same reasons mentioned before by others. Doubts and eagerness to point out how it might not work. Lack of vision, breadth and depth to understand the problem and implications. Lack of technical ability. Lack of time. Lack of motivation.
And after reading and talking to people, it seems like ideas end up changing. It's only when you're a bit bigger that you'd have to worry about secrets.
Co-incidentally I wrote a post titled "Mere ideas have never been this useless" just earlier today. You can read it here: http://www.zaid360.com/?p=63
--Zaid
If your idea is good in a duh-obvious sense, then you can be damned sure someone else is already doing it or has done it. The way to succeed in this aspect is by doing it better where others have been lazy.