Backing up bitcoins on Mt.Gox I have some bitcoins on Mt.Gox. Against the possibility that a nuclear holocaust or some sort of hanky-panky or the like were to wipe out Mt.Gox tomorrow morning, how should I back those up?
Mt. Gox operates a hosted (shared) EWallet.   They claim to have cold storage and perform other backup practices, but there is nothing that you as a Mt. Gox accountholder can do to "back up" those funds.  And if there is a problem restricting you from accessing your Mt. Gox account, there is nothing you can do to recover those funds.

If you have your own wallet, perhaps running the Bitcoin.org client, you then do have the ability (and responsibility) to back it up.