Clients publish transactions to each other within seconds. If a new transaction is not a copy of the short term list of new transactions and is not a copy of something in the block chain it is probably ok to believe it will be accepted and you can proceed with the transactions.
The risk is that your client hasnt seen the duplicate and the other transaction gets incorporated in the chain. I believe the chances of that happening for a normal transaction are low. The double spender would have to actively attack your client network connection to accomplish a double spend and for normal transactions a wait of a few seconds is perfectly fine. We would need the window to be large enough that we are very confident most clients have seen the transaction yet short enough that the selling merchant can wait without burdening normal customers. 15 seconds for example.
For high value transactions (like paying for a car) waiting for a few confirmations seems acceptable.
I think the merchant's client could be modified to let the merchant know there are no conflicting transactions in the active list after 15-30 seconds.