[rfugger]

A 51% attack only gives you the ability to double-spend coins you already control. It doesn't give you the ability to crack the private keys of others. So there is no straightforward economic incentive for a 51% attack unless you own a huge amount of Bitcoin, and thus would be hurting yourself.

[Retric]

You can double spend with wallets you own so 1BTC > 2BTC > 4BTC ... any number you want fairly quickly.

[dllthomas]

I think this is false. Double spending doesn't create coins, it screws someone who accepted a coin. My understanding of the 51% attack is that you can basically reliably snip the end from the block chain. That doesn't let you put arbitrary things into it.