They can buy cheap, dump high, catch it back at the bottom.. and they're experienced, they know how to do this over and over.
They can cause enough volatility that none of us will feel safe, the newbies will panic, the prices will sway around, no stability will cause colossal reputation problems with this fledgling currency.
Am I wrong here? This is David vs Golliath, except David is a newborn baby in this story.
Put a buy in for 10,000 BTC at .01 USD on Gox, then you're covered. And yeah, a billionaire could *temporarily* screw the market if he wanted to, but the most important thing to remember is this -- Mt. Gox is NOT the same as Bitcoin.