Would it be possible to manipulate the exchange rate of Bitcoin, if a single entity controlled the majority of all available currency?
Well, everyone can manipulate the exchange value of a currency by trading large volumes.
Having 51% in particular does not really change that.
Besides, for lowering the price, he will have to sell a lot, probably losing his 51% position.
So basically, you can't manipulate the price just by having > 51% of the bitcoins, but by trading very large volumes. Someone with very much fiat currency can also manipulate the price by buying large volumes.
To see howmuch you will have to sell (buy) to make the price go down (up) a certain amount can be seen in the "market depth" of the exchange. For Bitstamp f.e. the market depth can be found here. The steeper the market depth curve, the harder it is to move the price in that direction, so currently (see picture below) it is easier to make the price drop by selling bitcoins than it is to raise the price by buying.
The "51% attack" that is a known vulnerability impacting proof-of-work digital currencies like Bitcoin has to do with 51% of the mining capacity, not 51% of the currency.
There is a concept referred to as the "economic majority" but that describes how those who hold coins and buy newly mined coins are the parties who essentially have the final say on whether changes to the protocol will be accepted.
As far as a party with a lot of coins having the ability to impact the exchange rate, that is true. This is true though whether the commodity is bitcoins, gold, pork bellies, or any other commodity. With the ability to open a short position (borrowing bitcoins and selling them), the exchange rate can be manipulated without even owning bitcoins. Now whether this activity is profitable or not is a completely different matter and has to do with liquidity, time frame, tolerance for risk and other factors specific to the speculator(s) attempting the manipulation.