What has happened to Mt. Gox and $375m of Bitcoins?

It was once the biggest Bitcoin exchange in the world, accounting for as much as 80% of the global trading market in the virtual currency, but Mt. Gox suddenly stopped trading yesterday and reports suggest that some $375m worth of Bitcoins, or 6% of all Bitcoins in circulation, have gone missing. In an official statement, the Mt. Gox management team state that: "In light of recent news reports and the potential repercussions on Mt. Gox's operations and the market, a decision was taken to close all transactions for the time being in order to protect the site and our users. We will be closely monitoring the situation and will react accordingly."