as long as ghash.io got over 50% of the network's hashing power they can reverse every transaction since 7th of jan 2014.
Well lets keep the hyperbole to a realistic level shall we.
1) With 51% of the hashrate the main chain will continue to grow at a rate of 71 blocks per day and the attackers chain would grow at a rate of 73 blocks per day. So to reverse a t x which is 279,000 blocks deep in the chain with a 2 block per day advantage would require about 35 years.
2) While the attacker is building the attack chain they won't be mining on the public chain. Usually an attacker would keep this a secret until it the attack is complete because it ensures there is no warning but what this means for a public pool is that the number of blocks on the main chain produced by "ghash.io" would be 0. Miners would receive 0 BTC in compensation. GHASH.IO share of new blocks would drop to zero and their share of the network hashrate as charted by various sites would also plummet to zero (because those sites are based on detected blocks on the primary chain).
So putting these two things together ...ghash.io would need to attack the network for 35 years and not have any miners leave despite not getting paid and the fact that it would be very obvious what is going on (i.e. if you have x PH/s and are producing no blocks on the main chain then what are you doing). It isn't good for any entity to have that much hashing power but there are practical limits on what can be achieved by a pool.
Thank you for posting this. I just bought my first coins this week and put them into cold storage. These threads were kind of spooking me about my investment.