How I understand this issue

I'm going to intentionally use made up numbers to make the math easier. Concepts should remain the same. Also going to use http://en.wikipedia.org/wiki/Alice_and_Bob terminology.

Imagine combined honest rate is 100. That would mean Eve needs at least a rate of 100+ herself. If she has that, she could announce a bitcoin transaction for buying something from Bob. At the same time she announces a different transaction to her own mining pool. Both the honest pool and Eve's pool start computing their block chain. At some point Bob will accept the transaction as verified. Let's say this occurs at block x.

At this point, three possibilities exist: Eve's block chain could be longer than the honest one, it could be equal, or it could be shorter. Depending on the ratio of Eve's rate vs the honest rate, one of these cases will become more probable, but the chance exists it will either of these.

When Eve's block chain is longer than the honest chain, her attack is complete. She announces the longer chain to the world, the world sees a longer chain and believes this is the correct chain and continues as normal.

When Eve's block chain is shorter than or equal to the honest chain, she can continue to compute until she has a longer chain. If she has more computing power than the honest pool, she will eventually reach such state. If she has not, her attack will fail.

Eve could change all transactions she sends out without an extra cost during the period of the attack.

So what does this mean: this attack is profitable for Eve from the moment the value of all her transactions combined are greater than the cost of running a mining pool. So Bob should wait until the transaction to him is verified by enough blocks so he believes the cost of running an attacking mining rig is greater than the gain from reversing the transaction.

Since the cost of running a mining rig is somewhat expensive, most transactions will not be bothered by this kind of attack. When Alice and Bob do receive transactions from Eve that are worth reversing, that would probably mean Alice and Bob have some huge resources as well. Since they are relying on Bitcoin infrastructure for large transactions, they should be mining themselves to protect their transactions.

Monitoring when such an attack occurs is quite trivial I believe, so we would know when it has happened. We would also know which address(es) were sending the revoked transactions. In a lot of cases, these could possibly be traced back to who owns them. The people who receive the money probably know already. Proving it was them would be relatively simple. That would mean that, should a legal framework exist, it would be quite easy to punish Eve accordingly.

Therefor I believe this hack is very unlikely to happen.