When Satoshi argues in his paper that Bitcoin can't be subverted unless someone controls over 50% of the computing power of the Network, he is thinking of an equal distribution of generating power amongst hundreds of thousands of nodes.

But what has actually evolved is a bit different than that.  As the difficulty increased CPU generation became no longer practical, so mining was pretty much confined to a small number of nodes mining on multiple GPUs. 

Then mining pools evolved, and the small number of pool operators controlled generation, not the pool contributors. 

People can switch which mining pool they contribute to almost instantaneously, and it is not beyond the realm of possibility that a pool operator could suddenly offer a great deal that would motivate over 50% of the generating power to switch to his pool, and fork the block chain.

This is a much greater risk than the Sybil attack, and could actually happen.