Quote from: angelamerkel on January 21, 2014, 04:21:13 AM

Good point. The attacker could do that, however transactions that are older than 1 hour could never be reversed! And isn't that the whole point of an 51% attack?

The only thing an attacker could do is now

* prevent any transactions from happening
* reverse or double spend transactions that are younger than 1 hour.

which is very expensive and not that dangerous imo.

If a Merchant receives a big transaction he could just wait 6 confirmations (maybe double check transaction on blockchain.info and blockexplorer.com) and even if the attacker is in full control for the whole time this transaction could not be reversed!

No, if they make a concurrent fork then they could distribute the block to many nodes in the network after they have confirmed that they node they want to attack has received the transaction they wish to reverse.