One more thing: if you are looking for more of a partnership rather than commissioning something, there may be other things you can bring to the table. Are you an awesome graphic artist? If so, you might be able to find a programmer to work with you — designers are really hard to find. Likewise if you've got tons of experience starting small businesses and finding venture capital (and it has to be experience, not just "I think I could do that").
My usual metric for considering a partnership is that an idea is worth a few drinks. A full specification (with wireframes showing the flow between screens, definition of the project scope and where the data comes from, and maybe a bit of artwork) is worth a bit more. If you also have some market research done systematically and written up, an analysis of the competition, an ability to state your unique selling point in a couple of sentences, an appreciation about what sorts of thing are difficult to code, and some ideas for a marketing campaign (including how you'll fund it), you're getting there. If you have all that plus a business plan, evidence that you've already approached the bank and/or some venture capitalists, some thoughts on where you'll get the other people the project needs (designers etc), some form of risk analysis and some thoughts on what happens if the project bombs, now we're talking.
If the app is of the sort of scope where one programmer working three hours a day would take three months to write it, you'd have to show how you'd expend similar effort to make the project a success. Too many people seem to think that their work is done after coming up with an idea (which takes hardly any time and isn't worth that much), maybe spending one evening developing it, and then just letting someone else do the coding and expecting 50% of the profit. I'm not saying you're doing that, but programmers get that kind of offer really frequently, so it makes them quite wary.
