What if someone owns 51% of all bitcoins? Would it be possible to manipulate the exchange rate of Bitcoin, if a single entity controlled the majority of all available currency? 

http://en.wikipedia.org/wiki/Currency_intervention
http://www.nber.org/papers/w14600
http://ask.metafilter.com/38377/does-51-sharehold-mean-control
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.