I agree that it would be neat, but I fear that the reason only big banks and credit card processors have attempted to dabble in this space is because you've just described a bank. Building one of those carries a whole host of regulations, even if the goal is a very simple, limited set of interactions.
It works for Virtual Worlds though. At this very moment I am dancing at a party in InWorldz, and I just tipped the dj with Inworldz dollars that I previously purchased with USD. If she wants to withdraw the money as USD she can.
