Someone would have to gather 50% of all the coins first, assuming coins were fairly and broadly distributed this could be very hard to do. As you buy more coins, the price increases, the more you buy the more extreme the price increases become.
I worked it out once, to buy a 50% of Bitcoin supply (imagine it was a PoS coin) would cost far more than buying mining equipment for 51% of the hashing power.
Also in PoS once coins are used to stake they are unable to stake another block for an amount of time (they become locked) so you would have to split the coins into many addresses probably hundreds to be able to do the attack. That also means the rewards are locked for hundreds of transactions, which makes it nearly impossible to create a fork and spend the funds before the network responds.
SolarCoin, Gridcoin and Pinkcoin are wirth a look if you are interested in PoS.