Gold dropped 9.4% a few days ago. That had little to do with what the majority thought instead people wanted to sell slightly more gold than people wanted to buy and the price dropped.
Ass to messing with the block chain, the real problem is a 51% attack is one that people don't know for a while, start randomly seeding bad transactions and there is no obvious point as to which where good transactions and which where bad ones. People my suggest reversing all transactions but if you traded bit-coins for stuff your now out your bit-coins and your stuff. Net result Bitcoin's probably dies.
Absolutely true. "Introduction of double-spend and other corrupt transactions because you control a significant fraction (perhaps LESS than 51%) of the mining power" is a vulnerability for bitcoin that is NOT present in other forms of value.
But "a majority of the users could redefine what is considered valuable" is a vulnerability (mentioned in the post I was replying to) of EVERY representation of value, and not a particularly interesting or realistic one.
Ass to messing with the block chain, the real problem is a 51% attack is one that people don't know for a while, start randomly seeding bad transactions and there is no obvious point as to which where good transactions and which where bad ones. People my suggest reversing all transactions but if you traded bit-coins for stuff your now out your bit-coins and your stuff. Net result Bitcoin's probably dies.