What about the concept of detecting double spends in bitcoin nodes and notifying merchant? This has been proposed before, and was just having discussion with someone from Bitsimple on this (they suggested it). They were saying you as a merchant or more likely a service the merchant is using, can deploy a bunch of nodes across the globe (not a huge number, but maybe 10-50). Merchant get's your transaction and does wait like 10 seconds, but no more. If any of the service's nodes see a second transaction trying to spend the same bitcoins, it notifies the merchant POS/wallet system. If no second transaction is seen in 10 seconds it is considered 'safe'.