A few months back, Apple ran into some trouble with numerous complaints regarding iPod Nano flaws. At that time, the screen was very easily scratchable, and even carrying it around in your pocket could ultimately render it unreadable. A similar problem is plaguing them now, though much less serious in nature. Over time, the Nano's color will fade or change, resulting in a much less pretty device. Apple is aware of this, and in fact have even imprinted the box with that very statement!
So why does the Nano's color fade? Does the ink fade? Does the oil inside the plastic separate from whatever dyes they use? It seems unusual, considering most plastic and metal enclosures I have pretty much retain their color for the life of the product. One could conclude that perhaps Apple did this intentionally. Personally, I'd much rather see my iPod retain its color. Regardless of what Apple says, a Nano is not a pair of jeans.