I really don't like the idea of setting it up as one organization overseeing everyone. We don't need that kind of centralization IMO.
What about promoting an Intrade-like site to bet on defaults? You could have bets like "each share pays 10 BTC if MtGox defaults within the next 30 days" with a floating price, where people can either buy the shares, to insure themselves, or short them, if they think the price is above what the risk of default merits and think they can expect to earn a profit.
I think it could work for sites where account holder can expect to earn a profit, like GLBSE and Bitcoinica.
I don't see this as a central body, it wouldn't be "overseeing" everyone. It's just a membership organization and would be totally optional.
There are significant benefits to giving users peace of mind that their Bitcoins will still be there tomorrow if there was a break in, and that other organizations have fully vetted you & your practices and believe them to be within industry best practices and standards.
I'm not quite sure I follow on the betting aspect. Can you elaborate further?