Dear Hackers,
Here is the challenge:
Please create the next killer app, and give all the rights to me so I can take all of your work and make a bundle of cash. I know you like building things, and I like making money. Seems like a good arrangement to me. I might even throw in a free T-shirt.
I would relish the opportunity to meet new people and work on something different.
Every programmer I know have more ideas than they will ever have time to finish. Why would they do something like that?
I've run one hackathon myself and it most of the feedback I got was quite positive. Off the top of my head, the things that people liked were; the chance to meet/work with new folks in a low-risk way, to try out some new tech/language/framework that they'd never got round to, do something within a short deadline.
Overall, it went pretty well and it I don't think anyone was (or felt) 'used'.