[perseusmandate]

If you have amassed enough hashpower to be able to do the 51% attack you have far more to gain economically from continuing to mine for the network than to somehow short it (impossible to find a counterparty for a transaction that large) and then have billions of dollars of hardware that is now worthless

[imtringued]

Also if multiple parties attempt to mount a 51% attack then they will have to compete with each other which makes the attack impossible.