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jimrandomh comments on [tangential] Bitcoin: GHash just hit 51% - Less Wrong
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51% of hash power only grants the power to roll back recent transactions which you sent. It does not make it possible to enter invalid transactions, to roll back transactions you weren't party to, or to steal coins at rest. The risk is that you could receive coins, do something in response to receiving those coins, and then discover that they were clawed back. But the further back in time the transaction was, the more computationally expensive it is for them to do this.
While it doesn't allow invalid transactions, it does enable rolling back other people's transactions, by a combination of rolling back time and rejecting a class of transactions, such as a particular address. In particular, it allows ignoring all other miners and taking all the newly mined coins.
It's true that the further back in time you want to rewind, the more computational resources. In particular, the further back in time you want to go, the more time it takes to accomplish the maneuver. But if you are a consortium of miners, you were going to spend these resources mining, and the total number of blocks is fixed, so does it cost electricity? I'm not sure.