BullionVault does allow customers to exchange gold directly between themselves, using the GoldVault trading platform as an exchange. It wouldn’t take much for them, or someone like-minded, to attach a plastic-card issued by Visa or Mastercard to this, allowing the debiting of gold from BullionVault (or whatever the service is) at the user’s request.
I can transfer "digital" gold units to commercial or individual account holders (or vice versa) using my Goldmoney.com iPhone application. (See my thread: The Potential of Goldmoney.com’s new iPhone Application) The technology to do this is here now. The only thing lacking for this technology to take hold is a financial crisis, and I think that is only a matter of time.