This is not really a post concerning about mybitcoin, but about the future of the bitcoin economy.
The service like mybitcoin is no doubt to play a very import role in the future. I guess we need audit firm such as KPMG, PWC to audit the asset which these service provider should have. So is it possible for an audit firm to check whether mybitcoin has those numbers of bitcoin, but at the same time, preventing the auditor from copy the private key that mybitcoin have?
It's not only possible, it's trivial. Proving you have a particular private key is one of the simplest things to do.
But at the same time, you have to prevent STEALING in the process of the AUDITING.
The database of the mtgox was lost in the auditing process.
It's not like the auditing of gold storage. An auditor are not that easy to take away a piece of gold without being caught. But a private key is only a piece of information, the auditor got the information and got the money!