I'm always amused at the "why don't you try building it yourself" response developers give when they're asked this kind of question. The amount of knowledge you need to execute an idea is staggering. Deep down they know there's a 99.9% chance the guy will fail. It's as if a person went up to an architect and asked him to build him a hotel with promises of sharing the future revenue stream. Architect responds "I can't…
I've seen people learn to code in order to build their ideas. It's even occasionally turned out OK. I've also seen people learn to build in order to build their own gazebo, deck, and even a log cabin. It's not a hotel, but it is a useful piece of construction that a person with no architecture eduction can build with only a moderate amount of knowledge and experience. Some development projects are the equivalent of a…
That's the difference between an "ideas man" and an entrepreneur. And it's the easiest way to successfully differentiate when someone approaches you with an idea.
A real entrepreneur understands that ideas are worthless; only implementations have any value. Thus, they will be ready and willing to pay you (gasp!) for your time.
I, like almost every programmer out there, I'd imagine, have been approached multiple times by people with great ideas. What quickly distinguishes the serious partners is asking them what they can bring -- not will bring, but can bring -- to the partnership. A true partner will show you a rough mockup of their ideas, market research, and a stack of cash they pulled together. The ideas guy will tell you that the idea alone is worth your time.