The standard client prioritizes TXs by the time of receipt, e.g. a conflicting transaction will not be relayed. This is not enforced by the network, though. Is there anything that would prevent this scenario from happening with 0-confirmations?:
- Send 0.01btc fee TX to 0-confirmation accepting merchant.
- Receive digital good.
- Send 0.02btc fee TX directly to "all" the major pools/miners.
- Miners will include the higher fee TX because they make more profit from it. (Not sure they currently do that but they could.)
- brag about successful double spend
Is there anything but worry about their reputation keeping pools/miners from dropping lower fee TXs in favor of higher fee TXs?