Won't you just steal my idea?
First of all, I already have another (private) wiki filled with dozens of my own ideas that, if I ever get a break in contract work, I'd like to develop on my own. While I'm certain that your idea is a fine one, I'm really rather attached to my own app-ideas and would put a higher priority on developing them than on stealing yours.
Secondly, no offense, but your idea isn't that original. There are a gazillion people out there thinking of cool things to do with the iPhone and it's a fair bet that some of them have thought of your idea, already. Heck, it's a fair bet that my friends and I have already discussed how cool it would be if someone would do your idea! (We spend a lot of time jabbering about cool things you can do with the iPhone.)
But the thing is: ideas just aren't that valuable. We wish they were -- we WISH that someone would pay us truckloads of money just for our smart ideas, but the sad truth is that just about every business on the planet is more interested in people who produce something. So, as brilliant as your idea might be, the real value is in the execution of that idea -- the actual production of something. As I said earlier, all my spare time will be devoted to the development and production of my own app-ideas; I won't have time to steal yours.
Thirdly, and most importantly, it would be Wrong!
All that said, we'll be signing a mutual Non-Disclosure Agreement (NDA), which provides you with legal protections, early-on in the process.
One of my favorite bloggers has an article about ideas and their value. "No offense", but I think he's spot-on.