I'm not sure how much synergy there actually is between "get a taxi on demand" and "deliver goods on demand." I mean, taxis have existed forever. Presumably, if someone thought it was cost effective, they could have used taxis as a logistics network long ago. But taxis are really, really, really expensive (and Uber black cars are yet more expensive) in terms of what people expect to pay for shipping. UPS or the postal service will ship a small package across the country for me for $10 or less. A taxi in SF won't go a mile for $10.
Is there some reason to believe that Uber changes the game with regard to all that? They aren't drastically lowering the price of taxis. Their success has been in raising the service level of taxi-like-services, and in reducing the inconvenience of getting a taxi dispatched to you. I don't say this to take anything away from Uber -- I think they have a fantastic service -- but I don't see their successes as being things that really have a lot to do with what you want from a goods distribution network. If what you want to do is have a very expensive courier service for some kind of physical good, do you really mind calling dispatch and talking to a person?