Sell sell sell!
Sell what?
Everybody just withdrew all their Bitcoins. All that is left in the exchange is Dollars...
Per Bitcoinmonitor, at 15:30 UTC (80 minutes ago) the "big wallet" showed a balance of 49,900 BTC.
It's been taking consistent hits since then, so I assume it belongs to MtGox. Just now, 16:50 UTC, it showed as 44,386.
I assume from the round figure of 50,000 BTC start (nearly) this is not their entire stock of BTC.