Thank you, I fully agree with your opinion, but I suspect the actual buying and selling is all done on a SQL database, and the only time the actual Bitcoin goes into play is when someone asks for a withdrawal. I think this method is a good idea, since it saves money for everyone. Consequently, though, 500k Bitcoin has been "stolen" may very possibly just be an arbitrary number in an account, which means nothing more than an IOU from MtGox.