I am just going to add on to the speculation a bit here. There has been a confirmed leak of audio from January with Mark and a bank of theirs in Japan. This bank was going to shut them down. Without a bank mt gox would have to stop operations and there would be withdrawals in mass. The shit would hit the fan if they were operating under fractional reserve. Is it just a coincidence then that shortly after mark finds out his bank is shutting him down that they lose several hundred thousand bitcoins due to "malleability" ?  I don't think so. Mark is using the malleability as a scapegoat. I think he was operating under fractional reserve and I also think he tucked away many bitcoins for himself since his exchange was going down anyway.