If you have a white Apple Laptop (MacBook, ibook, ect…) and it is showing signs of dirt the Macosxhints web site has a few tricks to help clean it up. The main article says that an eraser will work real well. I would stick with a white eraser the standard pink or many other colors out there i would think could turn the color to the color of the eraser. A few of the commenters say that the Mr. Clean Magic Eraser real well.
There is one other option. Apple has said that some of the laptops are going to get the white palm rest area replaced for free. To find out if your white laptop is one of the items that will be replace make an appointment with an Apple Genius or locate an Apple Authorized Service Provider.