They are already doing same day delivery in some cities right now with their vans and Im sure that as they tweak it more they will expand unless it goes terribly wrong..
http://www.amazon.com/b?ie=UTF8&node=8729023011
The one thing I don't know is how cost efficient is it really; to do it themselves? Think about the cost of vehicles, fuel, maintenance, insurance, registration, employees, benefits, workman's comp, disability and unemployment insurance. This list goes on and on versus paying a single shipping charge to UPS at a good discount Im sure to deliver it. Not to mention that amazon is a company that already does not turn a profit.
Amazon cares about profit no doubt BUT with that being said they care about customer loyalty almost as much. This is a company that will refund $2000 dollar items at the drop of a hat. All the customer needs to say is UPS/USPS/FEDEX never delivered it and almost instant refund. They would take a loss to control more in the process on how they get their items to their customers. If it costs them 10 billion in the short term to make 50 million with every day delivery you can bet your butt they will do it.