I don't want to sound against this idea because I'm not but I just don't see there being a lot of interest in this. I know coders and coders don't work for free (which is pretty much what this is), then even if their is a worthwhile prize not many coders will want to risk it on the off-chance that they might win something. Then there is also the question of where the design to be coded is going to come from?

Like I said I'm not against this idea at all but it needs a lot more planning and, personally, I would like to see a lot of interest from potential entrants before something like this went ahead (nothing worse than a failed contest).

Just needs some more thinking through, do you know of any places that already holds coding contests like this? Might be worthwhile checking them out and seeing how they do things, work out the best way to ensure interest.