With what happened at MtGox (fake trades pumping up the price, non existent money used to buy bitcoins), who is currently checking the largest bitcoin exchanges to make sure the same games are not being played again?
Ultimately it hardly matters. As long as at least so me percentage of people is actually paying real money for bitcoin at that price, it means there is demand and support at that price level. The ones that really will be hurt are the people who sell bitcoin at affected markets and recieve no money, because the owner of the exchange 'bought' bitcoin with fake money.
Sure the price will tank for a while when that happens. But it will not really have a permanent effect other than that a large amount of coins is stolen.
That's why you should not tryst shady exchanges.
And definitely hold your bitcoins in your own local offline or cold wallet. Don't leave them in an exchange.