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.