I went back to an Apple store in London yesterday with a dark spot imperfection on the silver aluminium back - I sat and opened 7 new 32GB white & silver iPhones out of new retail boxes and each one had varying degrees of imperfections ranging from etches to the bezel you could run your fingernail into, to multiple 'spots' on the aluminium back to sharp burrs of metal around the side cut-outs which had been machined poorly.
I gave up after 2 hours of looking for what I would consider to be an Apple iPhone of the quality I would expect out of the box. The Apple employee was great and helpful the whole time and apologised that I could only leave the store with a refund, not a perfect iPhone. I've never opened an Apple product before which wasn't perfect and this has been a frustrating and disappointing experience. The phones were from different week numbers so I assume different batches/factories.
This morning I phoned to check if there was an official line on whether these imperfections to the finish are a 'feature' of the phone which we should just accept of whether I should expect the level of finish to be the same as any previous Apple product I've ever bought. I was told that I needed to be happy with the product and could keep exchanging them, but that "no iPhone 5 is perfect, despite Apple trying their best". So I guess the message is you're welcome to search for a perfect iPhone 5 and the Apple guarantee will back you up 100%, but don't expect to find one very easily!
The first time I've felt let down by Apple, ever. I'm a massive fan of all their products and own most of them, but the phones I viewed yesterday felt cheap and tacky compared to the usual amazing end product I've come to automatically expect from them.