Not a legal expert by any means, but wouldn't the design count as intellectual property. I've heard talk of mailing yourself the documents (so you get a date put on the envelope by the USPS), then preserving the unopened envelope. That way, you could say that it was created by such a date, so if somebody else comes up with the same idea, you can show that your idea predates.
plus, I would imagine that the submissions would be mailed in, not posted online, with only the winning entry being posted-- and the intellectual property for that one has been addressed in an earlier post.
My thinking about this contest is that it would probably be better that it originate with some custom maker, or someone with some bench time, and that the materials be strictly limited, as to make the knife feasible to produce.
As much as I want adamantium in my knife, that would offest the plutonium content, and then yadda yadda yadda.
chizpuf