Is this actually possible? You couldn't change the original transaction without the private key, nor can you spend the results of it until it is in an accepted block chain block, no? Am I wrong?
You can broadcast a valid tx that depends on an unconfirmed valid tx that was already broadcasted. The second tx will only be confirmed if the first is, this is how betting sites that accept 0conf txs (such as SatoshiDice) work.
Not sure if pool software currently selects txs as a sum of the fees of the txs that depend on them, but it seems reasonable to implement.
