From the Amazon Shipping announcement: "Currently, Amazon Shipping is serving U.S. sellers with warehouses in fifteen metropolitan areas across the country with plans for future expansion."

Does anybody know what those 15 metro areas are? I posed that question to Seller Support, and this was the response:

We understand your curiosity and you are interested in knowing, but seller support is here to help you with any problems you have in your account, which means that if you don't have a shipment in process and you can't send us a screenshot, you must contact customer service, since the list is something to apply depending on your region, your state and even more important if you really are giving you that message seller central when you make your shipment to be able to us investigate, so if you have more questions, please contact customer service.