Suppose there's a transaction for 100,000BTC and several blocks later a double spend with 50,000BTC fee...
Maybe there should be a maximum fee per transaction.
Or maybe you should wait 60 blocks before considering a half-a-million-dollar transaction final if you think there is any possibility of a mining cabal trying to do this.Maybe there should be a maximum fee per transaction.
All of this reminds me, I need to clean up my user-defined-checkpoints code for the 0.7 release, so a cabal of merchants and exchanges can get together and decide they will "lock in" an agreed-upon blockchain after 6 (or 60 or whatever) confirmations. I think that would go a long way towards infrastructure for injecting real-world knowledge about who is trustworthy into the block-chain.