[Anderkent]

But it'd be difficult to keep such an attack secret, and the value of bitcoin would zero pretty much immediately.

So the only way to make money on this attack would be to short a very significant amount of bitcoin. I don't know it that would be illegal - does it count as insider trading if you cause the value change?

So the mining costs do not have to cover the entire value of all bitcoins. They only have to be large enough to discourage political attacks, and large enough that a bet to short twice the mining costs worth of bitcoins would be noticeable.

Still, it's an interesting point that I haven't seen raised before.

EDIT: woops, other siblings of this posts are right. The protocol validates all chains before accepting the longest one as true, so you can't print or steal bitcoins even with 51%