I suggested something similar (way later) here: https://bitcointalk.org/index.php?topic=196136

In fact, I think a reasonable solution could be implemented by adding a *single* instruction to the script language: suppose you have "OP_CHECKSIG_VAL val sig pub_key" which checks if sig is a valid signature of val under pub_key. Then you can create e.g. two transactions as follows:

• TX1: Take 1 BTC from TX0out, give 0.2 BTC to Addr1, 0.8 BTC to Addr2
• TX2: If you find a double spend of TX0out, give 0.2 from TX3Out to Addr1 and a fee of 10BTC to the miner

The buyer gives both transactions to a vendor who owns Addr1 who additionally checks that TX3Out really has 10.2 bitcoins and is still open. If at some point the vendor gets a double spend he can plug in the values into TX2 so that it becomes valid. Miners will be happy to mine a transaction with a 10BTC fee.

The suggestion in https://bitcointalk.org/index.php?topic=196136 is somewhat nicer because it increases the chance the vendor gets his money, but more complicated.