It looks like they have changed the plastics used...It looks like all the complaints about the white MacBooks have finally gotten through to Apple.
A small, but still significant number of people have been reporting an unnaturally fast discoloration of their new Apple portables, especially in the areas where the hands come into contact with the device. While initially passed off as improper handling by their owners, it turns out that there is indeed something strange about the plastics used in some models of the white MacBooks, that causes the discoloration, sometimes after not even two weeks of use.
Fortunately for those unlucky few that have this problem, Apple has officially recognized it as a manufacturing problem, after previously saying it was a 'cosmetic problem,' which is caused by the owner through improper handling, and which does not affect the functionality of the device and is as such uncovered by the Apple Care warranty.
Those with discolored MacBooks should contact AppleCare and inform them of the stain problem. The replacement top case should no longer have this issues, nor will the newest Macbooks as Apple seems to have changed the plastic, the newest MacBooks having a much smoother feel to them in those areas, while the ones with the problem feel rough.