Well, the answers posted by Mtgox are not sufficient. They still haven't answered how come 500.000 were stored by one single holder; which is very hard to believe. Either many accounts have been exploited, or more likely, it was mtgox's own account that stored this amount.
I think that most likely someone inside/outside mtgox hacked the site and has done away with far more bitcoins than mtgox is willing to acknowledge. Mtgox is furthemore palying for time by pushing forward opening time, as it does yet not know the full extent/full extent of the monetary value/or solutions to the lost bitcoins.
Probably more likely that someone hacked from the outside, as if it were an inside job it could have been done much smother without crashing the system. However, Kevin is more than very likely to be innocent as who in their right mind would be involved in a crime and then instead of just keeping quiet, announce that they have traded etc.