[ulrikrasmussen]

This would not be a double spend attack, but a denial of service attack. If you spend $600 million worth of mining work, you can prevent any transactions from executing for two weeks (although in this scheme where you pay twice the block reward to defectors, only one week).

[ohhhhhh]

Okay, sorry, I mean to find ANY flaw in bitcoin's protocol there is the weight of the entire market's worth hanging in the jeopardy. The incentive is to always be trying to crack it. So far, unsuccessfully.