To call Mt. Gox an “exchange” is rather a stretch. Mt. Gox is the roach hotel of exchanges. On a real exchange, you can buy as much as you want and sell as much as you want.
On Mt. Gox, you can buy in all you want. But when it comes time to sell out? $1,000 a day is your withdrawal limit. Oh, yeah! Let’s see. How long would it take to cash out on a one million dollar position? It would take 2.7 years.
As for bitcoin billionaires, it will take you 2,739 years to close out your position. There is a guy calling himself BitCoin Billionaire giving away lots of BitCoins. He probably did the math and realized he was screwed by the trolls who run Mt. Gox.
See why I call Mt. Gox a roach hotel? You can check in, but you can’t check out. (2014 edit: Sadly, Mt. Gox has shut down and a whole lot of bitcoin boodle disappeared too. So I spoke more truly than I knew.)
Which brings up a question. When you buy on Mt. Gox, whose BitCoins are you buying? Answer – you don’t know. My bet is that you are mostly enriching the owners of Mt. Gox. Why else would they have a $1,000 per day dollar/Euro withdrawal limit? I suppose that it could be that they wanted to make BitCoins climb higher by keeping people in the BitCoin fold.