Well, you could work with SMPAKE, which will credit your Mt. Gox account after one confirmation (as long as your Mt. Gox account is verified, 2 confirmations with no verification.)

  - http://bitcointalk.org/index.php?topic=102176.0

So simply obtain a Bitcoin address for payment from SMPAKE and give that to the customer.

That cuts your problem down significantly.

If you want 0/confirmations, just make sure the merchant's node is configured properly (no incoming transactions, explicit outgoing connection to well-connected nodes) and possibly offer to insure by assessing a small fee on each transaction that covers the cost should any race attacks or Finney attacks be succesful.