(I may have horribly misunderstood PoS, so be gentle!)
Reading about PoS, the general idea seems to be that each miner has a "hit rate" proportional to their coins / total coins in the system.
Now, if there is a reward (generation, fees, etc.) for successfully mining a block, we seem to have a positive feedback loop. A richer miner has more chance of solving a block and so their wealth would increase at a greater rate than a poorer miner.
Does this means that the richest miner in the system only needs to keep mining till they are rich enough to mount a 51% attack?