@thomasr As someone who was once very protective of ideas (I actually filed a provisional patent for Fluther way back when), my best advice is: don’t bother.
In the Internet world especially, ideas are a time a dozen. The problem is not coming up with them, let alone having people steal them—it’s just the opposite: the challenge is how to spread your ideas. It’s all about execution and dedication. All that money and time you could spend protecting your idea would be much better spent working on your product.
Go ahead and blog about your idea. Yell it from the mountaintops. Remember, to steal your idea someone would have to be: smart, motivated, have a couple years of free time, be convinced they can do it better than you, and finally think it’s better than the hundreds of other ideas they no doubt have. It just almost never happens. Plus, if it really is a good idea, there are probably at least five other people already working on it anyway (don’t be disappointed when you learn this)—that’s actually a good sign.
I’m amazed how in the web world now everyone just openly talks about the big projects they plan to work on, even in the earliest stages. No one has the time or inclination to steal these ideas. If anything, they’re likely to provide help or connections down the road.
So just go ahead and register your domain and get to work. :) And good luck!
I know I didn’t exactly answer your question, but I thought it’d be useful advice given I’ve been in your shoes.