No evidence, I'm afraid (I aspire to be a good enough UX designer that someday I MAY be good enough to work for Apple) but I can offer a couple of observations that, as Sherlock Holmes might state, leads one to believe that they very much engage in user testing.
First, a colleague of mine recounted that when Apple was designing the Newton many many years ago the designers went out with a block of wood with paper on it… they "used" said block of wood to sketch out ideas about what they would do with a mobile computing device in their day to day lives. Not user testing per se, but definitely user-centered design. While the Newton failed, I think it was more an example of the technology not being ready for prime time… Technology that could not match the vision of the design team.
Second, the final point that Steve Jobs has now made twice in two separate product announcements – that Apple focuses on the alignment of technology and liberal arts. They make products that are based on what people want, need, desire… Products that enhance their lives, and delight without detraction.
The idea that Apple designs such products without actually TALKING to users is to me an absurd notion. They are successful precisely because they focus smart and incredibly talented people to focus on people – not hardware. One of the best ways to understand what people want? You talk to them. You may not actually do formal usability testing per se, but I'd wager there is a huge contingent of professionals at Apple who do what I have done often – contextual inquiries, interviews and ethnographic studies into what people want.
Finally, the proof is in the pudding. Take an Apple product, software or hardware, and do a hueristic analysis. Do your own usability test, and ask participants to accomplish specific tasks. Apple will not (or potentially ever) get 100% success in the tasks you test, but I'd wager that most of what you test will be quickly and easily understood, used, and well received. Those controls and interaction models are pretty crisp and clear and I'd be quite surprised – and very impressed – if the designers at Apple got to such a final product design without doing some level of user research and design testing.
(Now, the obligatory disclaimer: I don't work for Apple, though I own Apple stock. I own Apple stock for the same reason I own an abundance of Apple products… I appreciate believe and support what they are doing. I'm not an Apple "fanboy.")