I agree with Athenian.
Besides that, I don't think I'd like to go through weight loss in such a public manner. I'm willing to do almost everything if it's a bet, but some bets I wouldn't take in the first place. If someone did offer me cool cash to lose some weight in, say, a month, I think it would be a much greater incentive for me than actual health considerations. It would have to be a short-term project or goal before I'd be willing, though.