All of them will eventually turn a little brown where your hands touch it, that's just the problem with buying a white computer. But the defective ones had a problem where the plastic sheet on top had little pits in it that would trap dirt and oil and it became impossible to clean it off. I've seen examples in the Apple Store before and it was pretty gross. Anyway, they'll fix it for you. Some companies sell cleaning fluid, but Apple usually recommends a damp cloth. Anyway, if your MacBook doesn't have a defective cover you should be able to clean most of the dirt off with a cloth. You can try using a polish cloth designed for a trumpet (don't use a silver cloth though!) that you can buy at your local music store, that's what I use on my computer. I also have used eyeglasses cleaning cloths they give you when you buy glasses.
