So I've been reading lots of this form since I joined and haven't read about anyone asking about this scenario. After reading the Forbs article about the largest bitcoin wallet (488k'ish coins valued ~2.8mil) I got to thinking. While is value isn't over 51% of all coins what if he decides tomorrow he wants to cash out? What would be the process? Just go to mt.gox and tell them to wire you the money for them? How long would it take and would it require breaking up sales over various buyers?
Mt. Gox doesn't buy or sell bitcoins from anyone, it matches buyers with sellers. If the owner of 488K bitcoins wants to sell them he will need to find someone to buy them. You can look at Mt. Gox's orderbook to see how much people want to buy now.
If he tries to dump it all at once he will crash the price and not get a lot of USD for it. A sounder strategy would be to put up an ask at a low price and wait for people to execute it.
You can only sell what people will want to buy. I am sure there is not 488,000 buy orders out there at x amount of dollars.