Ideas are like assholes.
Most of them stink.
You don't think yours does.
Someone else already had the idea for my first brilliant blog post title. And he took it from someone else:)
So I added a few lines to make it unique. Then I stopped googling and started writing.
So that's my advice. Recognize you are not the first one to have the idea, do something new to make it unique. Then get going! You will realize quickly that it's not the idea, or even the execution on your idea, but the problem you are solving that matters.
Paul Graham says, in one of his many genius essays...
"Actually, startup ideas are not million dollar ideas, and here's an experiment you can try to prove it: just try to sell one. Nothing evolves faster than markets. The fact that there's no market for startup ideas suggests there's no demand. Which means, in the narrow sense of the word, that startup ideas are worthless."
I have even seen this notion taken further. Not only are ideas not valuable, they can be a liability. I read a quote years ago (can't find it at the moment) that an idea is a $30,000 liability. Brian Sowards explains this here. Ideas deplete resources. Not just money, but brain power, time, social capital.
This is why it is super rare for someone to "steal" your idea. They hardly ever steal their own ideas! Eric Ries challenges readers to TRY and get someone to steal your idea... it's almost impossible (and even if they do, you might end up with 60m like the winklemorons:)
I opened one of my talks at LEANUF last week "What do Facebook, Flickr, iTunes, and the snuggie; all have in common? Answer, they were all my ideas." Gimmicky intro, maybe, but closer to the truth than you might think.
So... if you have an idea... accept that your idea is probably not original - so don't be afraid to share it, test it, prove it and turn it from a liability to an asset!