I think once you understand how the Order Book works this will make more sense. YES, Mt. Gox does have huge piles of USD and BTC, and we you buy 1 BTC for $100 on their site, no money changes hands, you're correct, just some account balances in a database get changed. But it's important to understand that the seller's balances are ALSO being updated, and everything needs to equal out. You have to consider that when you buy 1 BTC at "market value", you are fulfilling the order of someone wanting to sell at a value that they chose. So while no money is trading hands, I am in fact being paired with a real person who wants to trade.
For them to "control the price", they'd have to be falsifying the order book. They'd have to make it appear that someone wants to sell at a high price when in fact no one does. This would be very hard for them to do especially considering their trading data is open and free to use. I highly doubt that they can fake this: http://markets.thegenesisblock.com.
They don't need to provide an artificial price, they only need to provide some artificial dollar, those "checkbook dollar" would buy up all the coins for sell and raise the exchange price, thus attract more deposits
As Tirapon mentioned, in such a case many people would like to sell at a higher price, and the seller's balance will be updated with a lot of USD. But, as long as they can't withdraw USD, those account balances can be just numbers without real USD behind them
Actually this is the nature of any kind of centralized financial institution, be it an exchange or a bank: As soon as your money are deposited into their account, they will be mixed with the funds from many other accounts and the institution could practice fractional reserve, e.g. only holding part of the funds to satisfy some of the customers' withdraw request. So, if a large amount of users are requesting USD withdraw at the same time, they simply deny it since they don't have that much. This is the same for banks.
But anyway, bitcoin's supply is very limited, my estimation is that currently only a couple of thousand coins were supplied each day for sell (A small part of those daily mined 3600 coins and some amount of sell from previous bitcoin owners). If people's confidence of bitcoin increase, these little supply are not enough to satisfy the expanding user interest