First, to clarify, clients don't choose the longest chain; they choose the chain that took the most work to create. Someone could trivially create a new, longer

First, to clarify, clients don't choose the longest chain; they choose the chain that took the most work to create. Someone could trivially create a new, longer chain with the genesis block at it's root and completely rewrite the transaction history, but since they can't trivially create blocks that prove more work (as in proof-of-work), no other clients would use that chain. That said, clients will choose the valid chain that took the most work to create. If someone with 51% starts creating invalid blocks, nobody else will accept them, and the practical effect will just be a 51% reduction in mining power on the network. EDIT: Oops, brain fault. I had this tab open for a few days, then happened to come back, read the parent comment, and responded; only then did I realize it was posted 6 days ago, not 6 minutes ago..