I just wanted to point out that the finish on apples laptops is not paint, powder coat, or even bare aluminium. It is a layer of anodization, which is a layer of oxidation that is "grown" on the surface of the aluminium part.
This layer is harder than bare aluminium, reducing chances of scratching etc. It also makes the aluminium dyeable, as seen in some of apples other products such as iPhones and iPods.
Most users will do no harm to the anodization even after many years, but prolonged exposer to sweat can damage the coating over time.
Keeping it as dry as possible, and giving it a wipe down every so often will go a long way to keep it looking brand new for years.
My first gen unibody macbook's (late 2008) finish still looks like the day I bought it, ignoring imperfections causes by actual damage of course.
Hope this helps, cheers.