I think the design easily plays a part in anyone's consideration of Apple products across the spectrum...I won't lie when I tell you that my first Macbook way back when was purchased in part due to the fact that it was appealing to the eye in a design sense. I spent extra on the black model when the only difference was the hard drive and a negligibly faster processor...I spent extra because I wanted the black one. If the Macbook Pro's were available in the standard aluminum or white/black, and there was a premium price on the aluminum model, I would certainly pay it (as long as it wasn't absurdly different).
As far as the customer service being a legitimate reasoning for the premium...I don't see why it shouldn't be considered. I can tell you, without hesitation or question, that if a service was available in Atlanta for internet provider comparable to Comcast, but slightly more expensive because the customer service was vastly better, that I would switch right this second to the more expensive service. After purchase service, in my opinion, should be one of the primary focuses of consumer product companies...Apple just happens to offer (in my opinion) THE best service on the market.
I mean you take a Macbook Pro that is out of warranty, and you stand a very good chance of getting free or heavily discounted service on it if something abnormal goes wrong and is not the fault of the user. When that customer leaves with that positive experience, the next time they're ready to buy a computer, don't you think there is a VERY high likelihood that they're going to reference that situation and go "I'd pay $200 more with the idea that THAT level of service will continue to follow my Apple products."
To me, there's no question it factors into the premium...