There seems to be a new problem with the white Macbooks reacting to the sweat in their owner's skin:
See: http://discussions.apple.com/message.jspa?messageI D=2819568
and (self link) http://toxicsoftware.com/blog/macbook-tarnishing/.
I have some photos on Flickr
showing the damage. So far I've only found one other case like this - but i imagine there will be more. Whatever material Apple is/was using on the Macbooks they really need to test it better.