Yeah, but you also already have to trust the certificate authority, your ISP, your employees who have access to the server...the list goes on.
Furthermore, I believe the way it works is that clients make an SSL connection to CloudFlare, who makes another SSL connection to your service (using different key pairs.) Granted, they still get to see all your traffic unencrypted, but it all just comes back to trust.
SSL is end-to-end; the ISP is just one more untrusted carrier. And I would expect MtGox to have access controls around employee access to their servers. Both MtGox and their certificate authority have much stronger incentives to play nice than CloudFlare do.
Furthermore, I believe the way it works is that clients make an SSL connection to CloudFlare, who makes another SSL connection to your service (using different key pairs.) Granted, they still get to see all your traffic unencrypted, but it all just comes back to trust.