The price on MtGox was historically approx $100 higher than on other exchanges. Why? Because they had made it difficult to get Fiat out. So what did Mark do? He bought bitcoins on other exchanges and sold them on his own exchange. The price of btc is lower now on MtGox. Guess what he is doing now? Exactly the opposite. He is buying btc on his own exchange and selling them on other exchanges.
So people stop panicking and make this guy even richer. Keep your cool and hang on to your bitcoin on MtGox. Especially when they are below the market price as determined by the other exchanges.
The price will go up again and you will regret that you have sold cheap.