Well, regardless, your execution is now or imminently in competition with that of other developers who read about you right here.How many motivated, hard-working, capable people do you think are lurking here, just waiting for somebody to drop an idea in their lap? Wouldn't the motivated, hard-working, capable people most likely already have their own projects to worry about, and therefore lack the resources to jump on "your" idea? And do you really care about the people who aren't motivated, hard-working and capable?
Personally I wouldn't hesitate in the slightest to talk about my project(s) here... anybody who's terribly likely to represent competition to me, probably either already is, or would be so anyway. And the rest are just gonna read, go "uh huh" and move on to the next comment.
Heck, I'm so unconcerned about "idea theft" that all my source code is open-source, and under the Apache license.
And to avoid an accusation of hypocrisy:
http://www.fogbeam.com
http://code.google.com/p/screwpile
edit: to be fair though, what I'm building isn't a "site," it's enterprise software, although a SaaS offering could be in the cards down the road. So my case doesn't technically fit the OP's question as worded. But I still stand by the contention that sharing here is most likely harmless.