Someone figured out arbitrage. Profited off of Gox's inflated price.
That's not arbitrage. Arbitrage is taking advantage of different prices for the same commodity by buying at Place A and quickly selling at Place B and pocketing the price difference as profit.
Someone selling bitcoins at MtGox and then taking that money and buying more bitcoins at say, BTCe, quickly runs into a major problem: MtGox is taking weeks and sometimes months to process money transfers. By the time you have your money to buy bitcoins there is no arbitrage position.
Caused short term panic across other exchanges causing global drop in price.
That's just dumping with the anticipation that others will panic and follow suit. It's not arbitrage.
Prices back to normal by tomorrow.
Assuming the little artificial crash doesn't snowball with others speculators and investors thinking 'well, time to dump'. The price keeps falling as you buy back in, and continues to fall. Again, that's not arbitrage.
Wiki: "For instance, an arbitrage is present when there is the opportunity to instantaneously buy low and sell high." MtGox won't let you have the money you made, and there's a real risk the exchange will fold before they even think of giving it to you. Right now MtGox is refusing to even process many bitcoin outputs.
